Preheat the oven to 180°C. Grease the base of a 21–23cm shallow springform cake tin. Melt the chocolate in a heatproof bowl over a pan of simmering water. When it starts to soften, add the coffee and leave it for 2 to 3 minutes. Stir very gently. When the chocolate has melted, add the butter and stir until melted.
Meanwhile, beat the egg whites until stiff, then fold in the sugar. Remove the chocolate from the heat, stir in the egg yolks, then gently fold the chocolate-and-yolk mixture into the egg whites.
Mix the baking powder with the cocoa and flour, then sift over the chocolate mixture. Gently stir in the dry ingredients with a metal spoon. It should feel wobbly.
Scoop into the cake tin and bake for 35 minutes. Leave to cool in the tin, then turn out. Decorate with the marzipan roses.
To make the roses, roll out good-quality ready-made marzipan. Cut out circles and shape into roses.