Preheat oven to 175 deg C (350 deg F). Grease and lightly flour a mini bundt pan (12 small bundts or 6 medium bundts), tapping out any excess flour.

Place the chocolate pieces in a bowl, set it over a saucepan filled with some water, with the base of the bowl above the water level. Heat up the saucepan until water is just simmering, and gently melt the chocolate. When melted, remove from saucepan and allow it to cool. In another small bowl, combine whipping cream, lemon juice and vanilla extract. Stir to mix well and whip lightly until slightly thickened. Set aside.

Sift together cake flour, baking powder and salt. In a separate bowl, lightly whisk the eggs. Set aside.

In an electric mixer fitted with a paddle attachment, cream the butter and sugar until light and fluffy on medium speed (speed 3-4 on my Kitchen Aid mixer), about 5 minutes. With the paddle still turning, beat in the beaten eggs a little at a time, until well incorporated. Beat in the whipping cream mixture.

Reduce mixer speed. With paddle beating on low speed, add the sifted flour mixture and beat until just combined. Scrape down the sides and bottom of the bowl, and finish folding by hand.

Spoon out a third of the batter into a separate bowl. Stir in the melted chocolate and sift in the cocoa powder, and mix until well blended.

If using piping bags, spoon the batter into the bags fitted with plain tips (or if without tips, snip off the ends). Alternatively, you can use a small scoop spoon. Pipe or spoon vanilla and chocolate batter alternately into each mini bundt mould, fill to 2/3 of each mould. Use a bamboo skewer to swirl the batter just a little to create the marbling effect. Bake for about 20 minutes, or until a skewer inserted in the centre of the cake emerges free of batter. Cool the cakes for 30 minutes in the pan, then turn out to cool completely.