For cake, preheat oven to 160C. Beat butter, sugar and vanilla until creamy, add eggs one at a time, beating well after each. Sift flours with ½ tsp salt, fold half into egg mixture, add milk, then fold in remaining flour mixture. Spoon into a greased and lined 17.5cm x 25cm cake pan, smooth top, bake until golden (40-45 minutes). Turn onto a wire rack and cool.
For zuccotto filling, melt half the chocolate over gently simmering water and cool slightly. Whisk half the cream to soft peaks, add half the nuts and remaining chocolate. In a separate bowl whisk remaining cream to soft peaks, fold in melted chocolate and remaining nuts.
Line a 1.5 litre-capacity, 20cm bowl with damp muslin. Trim top of cake flush, then cut horizontally into three 5mm-thick slices. Cut an 18cm round from top slice, reserve. With longest sides facing you, cut each slice vertically into 3cm-wide rectangles. Halve rectangles diagonally to form triangles. Combine liquor in a shallow dish. Working with a triangle at a time, alternating between dark (the base) and light pieces, dip in liquor and place in prepared bowl with points meeting in centre. Ensure triangles are snug (trim to fit where necessary) and the dark sides are facing bowl sides. Spoon in cream mixture and spread evenly over cake. Spoon in chocolate cream mixture and smooth top. Dip reserved round in liquor, place over cream filling, fold over muslin, cover with plastic, weight with a plate and refrigerate overnight. Invert zuccotto onto a plate, dust with cocoa and serve immediately.