Sift together plain flour and corn flour. Stir in salt (omit if using salted butter) and mix well. Set aside.
In a stand mixer fitted with a paddle attachment or with handheld beaters, cream butter and both sugars until light and smooth. This may take about 3 to 5 minutes.
Add the egg and vanilla extract, and beat until well incorporated.
Tip in the sifted flours. Shift to the lowest mixer speed, and gently stir in the flour. Tip: Wrap a tea towel around the mixer bowl to contain the plumes of powdered flour!
Increase to medium speed and beat until the dough is creamy and smooth. If the dough feels too thick to pipe, add 1 tbsp of milk and beat until well incorporated. You want the dough to be pipe-able, but still thick. Note: The more milk you add, the more the cookies will spread too, so it is essential to chill the piped cookie rounds before baking.
Pipe the cookies and chill:
Fit a large piping bag with a large open-star or close-star piping tip. Fill the bag with a small amount of cookie dough first, adding more as you go.
Line baking trays with silicon or baking sheets. Pipe cookie rounds, spacing them 1.5 - 2 inches apart. Chill for 15 to 30 minutes. Remember - if you've added more milk, a longer chilling time is needed.Do not chill the cookie dough before piping, as it will become too hard and stiff to pipe.
Meanwhile, preheat oven to 175°C (350°F).
Place the baking tray with chilled cookie rounds in the centre of the oven. Bake for 10 to 15 minutes, or until the cookie edges just start to brown.
Once removed from the oven, allow cookies to cool on the tray for 5 minutes. Then transfer to a metal rack to cool completely.
For chocolate-dipped/funfetti cookies:
Melt the chocolate in a double-boiler or in the microwave. If using a microwave, place chopped chocolate or chocolate melts in a heat-proof bowl. Melt in 15-second intervals, stirring each time, until completely melted and smooth.
Once baked cookies are completely cooled, dip in the melted chocolate and sprinkle funfetti. Set on a baking sheet to firm up. This may take up to an hour at room temperature.