Heat oven to 180C/fan 160C/gas 4.
Butter and line a traybake tin, 23 x 23cm
or thereabouts. Put the chocolate,
sugar and butter in a pan, then heat very
gently for about 5 mins, stirring every min
until the butter and chocolate have
melted. Leave to cool for 10 mins. Beat
in the egg yolks, flour, vanilla and half the
Put the egg whites into a large, very
clean bowl, then whisk until they stand
up in peaks. Stir ¼ of the whites into the
chocolate mix to loosen it, then carefully
fold in the rest with a metal spoon. Pour
the mix into the tin, then bake for 25 mins
or until evenly risen and just firm to the
touch. Cool in the tin, then cut into squares.
Can be frozen for up to 1 month.
Beat together the cheese, vanilla, sugar
and remaining zest until smooth. Spread
over each chocolate square and serve. If
you’re making ahead, spread the topping
over just before serving.