Preheat oven to 175C / 350F. Line muffin tin with paper liners.
In a small bowl, whisk together flour, baking powder and salt. Set aside. In a separate bowl, whisk together cocoa powder and hot water. Set aside.
In a mixing bowl, beat eggs and sugar with an electric mixer on medium-high speed until the mixture is thick and pale. Turn down the mixer on low speed and beat in milk, butter and vanilla.
Sift in the flour mixture in two batches and fold in gently. Scoop 1/3 of the batter (about 1 cup) into the cocoa paste and mix well.
Drop alternate spoonfuls of vanilla and chocolate batter (in the ratio of 2:1, i.e. 2 spoonfuls of vanilla followed by 1 spoonful of chocolate) into the paper liners. Bake for 18 to 20 minutes until a skewer inserted into the center of the cake comes out clean.
Let the mini cakes cool completely on a wire rack before storing them in an airtight container. If weather is hot, store the mini cakes in the refrigerator.