Step1
Line a large square cake tins with baking paper,leaving a generous overhang on two opposite sides. Put the raspberries,sugar and lemon juice in a blender and blend to a smooth purée. Reserving 125ml (½ cup) of the purée,fold the remainder through the ice-cream and pour into the prepared tin. Freeze for 2 hours,or until firm.
Step2
Preheat the oven to 180C. Lightly grease a 30cm × 20 cm cake tin and line with baking paper,leaving the paper hanging over the two long sides.
Step3
To make the chocolate brownies,sift the flour and cocoa into a bowl and add the sugar,nuts and chocolate. Mix together and make a well in the centre. Pour the butter into the well,add the vanilla extract and eggs and mix well. Pour into the tin,smooth the surface and bake for 45 minutes (the mixture will still be a bit soft in the middle).
Step4
Allow the brownies to cool a little,then chill for at least 2 hours. Lift out,using the paper as handles. Trim and cut into 12 9cm × 5 cm rectangles then,using a serrated knife,cut through the centre of each brownie to form two thinner brownie pieces.
Step5
Remove the ice-cream from the freezer,using the overhanging paper to lift it from the tin. Cut into 12 rectangles,the same size as the brownies.
Step6
To assemble,put 12 brownie slices on a tray,top each with a rectangle of ice-cream and then another slice of brownie,cut side down. Smooth the sides of the ice-cream to neaten,if necessary. Freeze for 10 minutes to firm. Dust with icing sugar and serve with the reserved raspberry sauce.