To make the cake mix: Place the flour, cocoa powder, sugar, baking powder an salt into a large bowl and whisk until well combined. Store in an airtight container, at room temperature for 2 months.
To make 24 cupcakes/two 8 inch cakes: Place the cake mix into a large bowl and add the eggs, vanilla and oil. Whisk until the mixture starts to come together and then add in the milk. Whisk until well combined and everything is smooth.
For cupcakes: divide between 24 cupcake liners; placing 2tbsp of mixture into each liner and bake in the oven at 180C/350F for 18-22 minutes until risen and an inserted skewer into the centre comes out clean. Transfer to a wire rack to cool as soon as possible. Once cool, frost with desired icing and add sprinkles if desired.
For 8 inch cakes: Grease two 8 inch pans and line the base of the pans with grease proof paper. Divide the cake batter evenly between each cake pan and bake at 180C/350F for 23-27 minutes, until risen and an inserted skewer into the centre comes out clean. Leave to cool completely before filling with jam/buttercream and frosting the outside of the cake.