Grease and line a 15 cm (6 inch) square tin with nonstick baking paper. Grease the paper.
Melt the chocolate (see
Sift the flour and cocoa powder over the bowl. Add the ground almonds and stir in gently. Turn into the tin and level the surface. Bake in a preheated oven, 160°C (325°F), Gas Mark 3, for about 20 minutes until risen and just firm to the touch. Transfer to a wire rack to cool.
Cut the cake into 16 squares and, using a palette knife, spread a little mound of chocolate hazelnut spread on the top of each one. Make the icing by melting the plain dark chocolate with the syrup and butter until smooth and glossy. Separately melt the milk chocolate. Spoon a little of the plain chocolate mixture over each cake and spread around the sides with a palette knife. Using a teaspoon, drizzle lines of milk chocolate over each cake.