Coffee Chiffon Cake with Coffee Buttercream Frosting

Instructions

Preheat the oven to 165°C (329°F). Position an oven rack to the lowest in the oven. Ready a 20-cm (8-inch) chiffon tube pan. DO NOT GREASE.
Sift together the cake flour, baking powder, baking soda and salt into a bowl.
In a separate, large mixing bowl, whisk egg yolks and sugar until thick and fluffy, about 3 to 5 minutes.
Stir in the coffee solution, vanilla (or coffee) extract, coffee paste (optional), milk and oil, and mix until well combined.
Add the sifted flour mixture in 2 additions. Stir with a whisk until all the flour is incorporated and no streaks of flour are visible. The mixture should be thick, smooth and free of lumps.

Make the meringue:

Fit an electric mixer with a whisk attachment. In a dry, grease-free mixer bowl, pour in egg whites and sprinkle cream of tartar over.
Whisk on medium speed (speed 4 on my Kitchen Aid). When the egg whites become frothy, add sugar bit by bit in a steady stream. Whip until stiff peaks form. This may take 5 to 7 minutes, depending on your mixer and speed.

Folding the meringue into the batter:

Add ⅓ of meringue (whipped egg whites with sugar) to the egg yolk-coffee mixture. Fold in gently with a whisk until well incorporated.
Add another ⅓ of the meringue and again, fold in gently. Add the remaining meringue and fold in gently. The final batter should feel light, and have no visible streaks of meringue.
Pour into the chiffon tube pan from one place, and let the batter spread to fill the pan. Run a bamboo skewer gently through the batter. Smooth and level the surface with an offset spatula. Give the pan a few taps on the counter top to minimize air bubbles.

Baking and cooling:

Bake on the lowest rack in the oven for 50 to 55 minutes, or until done. DO NOT open the oven door. Only do so about 5 minutes before the end of baking, to test if done. The cake is done when a skewer inserted into the centre comes out clean. Note: A bit of cracking on the surface of the cake as it bakes is perfectly alright!
Immediately invert the pan once it is removed from the oven. Allow it to 'hang' until completely cooled.
To remove, run an offset spatula gently around the sides of the pan. Invert the cake to free it from the sides of the pan. Then run the spatula around the base to release the funnel.
Allow the cake to cool completely. Cover the sides and top of the cake with buttercream, and smoothen. Pipe rosettes as desired. Dust with sifted cocoa powder (optional).

Make the coffee buttercream:

In a small heatproof bowl, combine egg whites and sugar and whisk to mix well. Place the bowl over a saucepan or pot filled with 1 -2 inches of simmering water. Make sure the base of the bowl sits above the water level.
Keep stirring with the whisk until all the sugar dissolves, and no longer feels grainy between your fingers. Or keep stirring until the temperature reaches 71°C (160°F).
Transfer to a stand mixer bowl, and whip on medium-high speed until the meringue is stiff and no longer warm to the touch, about 5 to 10 minutes.
Add butter cubes, 2 to 3 pieces at a time. As you add the butter, the mixture will start to curdle and look awful. Keep adding the the butter and keep beating! After incorporating all the butter, keep whipping until the buttercream becomes smooth and fluffy.
Reduce the mixer speed to low. Pour in the coffee solution and whip until well mixed. Set aside in a cool, dry place.
Once the cake is cool, frost as desired.

Notes

Tips for baking chiffon cake

1. USE THE RIGHT PAN

Chiffon cakes are best baked in a chiffon tube pan without non-stick coating. Do not grease the pan.

2. USE THE APPROPRIATE PAN SIZE

In order for the cake to rise straight and tall, use the appropriate sized pan. When the pan is filled with batter, it should not be more than 3/4 full.

3. HAVE INGREDIENTS AT ROOM TEMPERATURE, INCLUDING EGGS

Cold egg whites do not whip up as well, so it won’t trap as much air as egg whites at room temperature. When blooming cocoa, remember to allow it cool to room temperature before incorporating it into the rest of the batter.

4. WHIP MERINGUE TO THE IDEAL STIFFNESS

5. SIFT POWDERED INGREDIENTS TOGETHER

This will give you a smoother and finer cake crumb and enable the cake to rise evenly and uniformly.

6. DO ALL YOUR MIXING, STIRRING AND FOLDING WITH A WHISK

The meringue will incorporate more easily and thoroughly, with minimal loss of volume due to the inherent shape and structure of the whisk. It also helps reduce or eliminate air pockets in the batter.

7. INVERT THE CHIFFON PAN ONCE OUT OF THE OVEN

Once removed from the oven, immediately invert the pan over a bottle neck once it is removed from the oven. Or if you have tube pan with ‘feet’, invert the pan over a cooling rack. Allow it to ‘hang’ in this position until completely cooled.

Tips for making buttercream

The butter needs to be cool and just softened at room temperature, but not warm. If the buttercream just isn't firming up after a few minutes of whipping, the butter could have been too warm.
Pop it into the chiller for 5 minutes, then rewhip with a whip on medium-high speed until it becomes silky smooth and fluffy.
If not using immediately, the buttercream can be transferred to a bowl. Cover tightly with cling wrap and store, chilled, in the refrigerator for up to 5 days.
Have it out at room temperature until it is soft enough to be re-whipped. When ready to frost the cake, whip the softened buttercream until it is smooth, light and fluffy again.