Decadent Black Velvet Cake

Heat the oven to 170°C fan. Grease and line two 20cm sandwich tins so the cakes release cleanly.

Whisk the flour, cocoa, bicarbonate of soda and salt together in a bowl, breaking up any cocoa lumps.

Beat 125g of the butter with the caster sugar until pale and fluffy. Add the eggs one at a time, then beat in 1 tsp vanilla.

Fold in the dry ingredients in three additions, alternating with the buttermilk. Stir in the black gel colouring gradually, then mix in the vinegar at the end.

Divide the batter between the tins, level the tops and bake for 28-32 minutes, until the cakes spring back and a skewer comes out clean.

Cool the cakes in their tins for 10 minutes, then turn out onto a rack and leave until completely cold before icing.

For the buttercream, beat the remaining 125g butter until smooth, then add the icing sugar in batches with the remaining vanilla and enough milk to make a soft, spreadable frosting.

Sandwich the cold cakes with a thick layer of buttercream, then cover the top and sides. Chill briefly if the icing feels too soft, then slice and serve.