Diana Henry's bitter flourless chocolate cake with coffee cream

Name: Bitter flourless chocolate cake recipe with coffee cream by Diana Henry
Yield: Serves 8
Ingredients: 160g unsalted butter, plus extra for greasing; 320g dark chocolate (70 per cent cocoa solids), broken up; 145g caster sugar; 5 large eggs, separated; 40g ground almonds; 300ml double cream; 1/2tbsp instant espresso coffee dissolved in; 1/2tbsp boiling water; 2tbsp whiskey, or to taste; 3tbsp icing sugar, or to taste, plus extra for dusting the cake