Sift plain flour and cocoa powder twice. Grease and line the base of a 21cm round cake tin. Preheat oven at 180 ºC.
Put butter, sugar and jam in a small saucepan over very low heat. Use a hand whisk to stir until butter has melted and the sugar has dissolved.
Place sifted dry ingredients in a mixing bowl, add in melted ingredients and stir gently to combine. Add the egg yolk and blend well with a hand whisk. Beat egg whites until just stiff, then fold into the cocoa mixture.
Turn mixture into prepared pan. Level up the surface with a spatula. Bake in preheated oven for 50 minutes or until cake is cooked through when tested with a skewer.
Cool cake in the tin before turning out onto a wire rack.
For the ganache topping: combine cooking chocolate and cream in a deep saucepan and stir over very low heat to melt the chocolate. Stir gently with a hand whisk until mixture is smooth. Leave aside to cool.
Slice off the top of the cake to yield a neat flat surface. Turn the cake upside down, then brush with melted jam.
Pour the ganache topping over. Use a long palette knife to smoothen the top so that the ganache will flow down the sides neatly. Sprinkle the top with toasted almond flakes. Leave the cake to set with the topping.