Preheat oven to 180 degrees C/Gas 4.
1) Prepare a cupcake/fairy cake tin with paper liners and lightly spray with nonstick spray. Sift the cake f;our, bicarbonate of soda, baking powder, and salt together. Set aside.
2) Melt chocolates and water over low heat in a double boiler.
3) In a mixer with a paddle attachment, slowly cream butter and 225g of sugar. Add egg yolks, 1 at a time, and beat until fluffy.
4) In a separate bowl, with a mixer and wire whip attachment, whip egg whites and 85g of sugar until stiff peaks form.
5) Combine milk and vanilla in a measuring glass. Fold melted chocolate mixture into butter mixture. Alternately fold the flour mixture and milk mixture into the butter mixture. Gently fold in the whipped egg whites.
6) Fill cupcake cups 2/3 full and bake until a toothpick comes out clean, about 15 minutes.