Black Forest Cake

Bloom the cocoa. Heat the oven to 175°C (155°C fan) for at least 20 minutes, and grease and base-line the two 20 cm tins. Whisk the cocoa powder into the boiling water until smooth, then set it aside to cool for 5 minutes. Blooming the cocoa this way dissolves it fully and deepens its flavour, so the sponge tastes richly chocolatey rather than dusty.

Mix the batter. Whisk together the flour, sugar, bicarbonate of soda, baking powder and salt in a large bowl until evenly combined. Whisk the oil, eggs, buttermilk and vanilla into the cooled cocoa mixture, then whisk this into the dry ingredients until smooth. Buttermilk's acidity reacts with the bicarbonate of soda to give the sponge extra lift and a soft, tender crumb.

Bake and slice the sponges. Divide the batter between the two tins and bake for 30-35 minutes until the sponges spring back when lightly pressed and a skewer comes out with just a few moist crumbs clinging to it. Leave them to cool completely in the tins on a rack before turning out, since a warm sponge is too fragile to slice. Once cold, slice each sponge in half horizontally with a serrated knife to give four thin layers; only three are needed, so the fourth can be crumbled for decoration.

Make the cherry filling. Drain the cherries, keeping the juice for later. Simmer 150 ml of the juice with the sugar, then stir in the cornflour slaked with a splash more juice and cook for 1 minute until the sauce turns glossy and thickened. Fold the drained cherries through the hot sauce and leave to cool; the cornflour sets the juice into a spoonable filling that won't run out of the layers.

Mix the soaking syrup. Stir 4 tbsp of the remaining cherry juice with 1 tbsp of sugar to make a brushing syrup. This keeps the classic sharp cherry flavour running through the cake, soaking into the sponge to keep it moist as it's layered.

Whip the cream. Whip the double cream with the icing sugar and vanilla until it holds firm, pipeable peaks, then chill until needed. A firm whip holds its shape both between the layers and when piped into rosettes on top.

Layer the cake. Brush the first sponge layer with the cherry syrup, then spread over a quarter of the cream and pipe a ring of cream around the edge to hold in half of the cherry filling, spooning it into the centre. Repeat with the second layer, then top with the third sponge and brush that with syrup too. The piped ring acts as a dam, stopping the juicy filling from oozing out of the sides.

Cover and decorate. Spread the remaining cream over the top and sides of the cake in a smooth, even layer. Pipe rosettes around the top, set a cherry on each one, and scatter over chocolate curls shaved from a block of room-temperature chocolate with a vegetable peeler; chocolate that's too cold will crack rather than curl. Chill the finished cake for 2 hours before slicing so it holds together neatly.