To make the sponge, lightly butter and line 3 x 20cm round cake tins and heat the oven to 180C/fan 160C/gas 4. In a large bowl, cream the butter and sugars together until light and fluffy. Add the vanilla and beat to combine. Meanwhile, in a separate bowl, mix together the flour, cocoa powder, baking powder and salt. Pour the coffee and milk together into a jug. Add the egg whites to the butter mixture, a little at a time, beating until fully combined before adding more. Add the flour mixture to the cake batter in three additions, alternating with the coffee mixture. Divide the batter evenly between the prepared cake tins and bake for 30-40 minutes, or until the cakes spring back to the touch and are coming away from the sides of the tins. Allow to cool in the tins for 10 minutes before turning out onto a wire rack to cool completely.