Method:

Preheat the oven to 180C
Using a hand mixer, beat the egg whites in a large bowl until foamy. Add the cream of tartar and continue beating until soft peaks form.
In a medium bowl, beat the egg yolks with a hand mixer. Add the sugar a bit at a time and beat until thickened. Add the yolk mixture to the egg whites.
Sift the cake flour, salt and baking powder into the egg mixture. Gently fold until fully combined.
Add the butter and milk to a bowl or glass measuring cup and microwave for 20 seconds, or until the butter melts completely. Add the butter mixture and the vanilla to the batter and gently mix.
If you are using DIY molds, grab a sheet of tin foil, fold it in half, then in half again, and, using a small spice container or other Twinkie-shaped item, mold the foil into an oblong shape. Trim the excess foil around the edges. Repeat to make more molds.
Fill the molds halfways and bake for 15 minutes, or until a toothpick inserted in the middle of the cake comes out clean.
If you are using a Twinkie baking pan, fill each indent halfway and bake for about 13 minutes, or until a toothpick inserted in the middle of a cake comes out clean. Let the cakes cool down.
Once the cakes are done, let cool.
Add the custard to a saucepan and mix the cornflour with a dash of milk. Add the cornflour mixture to the custard and heat whilst whisking to thicken. Once thickened slightly, allow to cool Fill a pastry bag with the custard. Use the tip to make 3 small holes in each cake, and squeeze in the filling.