Preheat oven to 190degC. Lightly grease and line two 20-22cm round cake tins with baking paper.
Use an electric beater to beat sugar, vanilla and eggs in a large mixing bowl for 3 minutes on high speed, until pale, light and fluffy.
Add milk, melted butter and a pinch of salt, then sift in flour, dark cocoa and baking powder and fold together using a large metal spoon until just combined – do not over-mix or the sponge will be tough.
Spoon mixture equally into tins. Bake for 15-18 minutes or until sponges spring back when lightly touched. Cool in the tins for 5 minutes before removing.
Measure flour by tipping it into cup. If you scoop it from the packet, you end up with too much flour!
You can make sponges a day ahead and keep in an airtight container in the pantry (not the fridge or they will dry out). If sponges do dry out, they can be revived with a drizzle of cream, liqueur or jam mixed with a little water.