Preheat oven to 180°C. Grease a deep 20cm cake pan and line base and side with baking paper. Dissolve coffee in 1/4 cup of just-boiled water and set aside to cool for 10 minutes.
Meanwhile, beat butter and sugar together in a large bowl of a stand mixer until pale and fluffy. Add one egg at a time, beating well between each until combined. Using a large metal spoon, fold in flour, hazelnut meal, milk and coffee. Spoon batter into pan and smooth surface.
Bake for 50-55 minutes or until a skewer inserted into centre comes out clean. Cool in pan for 10 minutes, then turn out onto a wire rack and cool completely.
Meanwhile, to make the caramel, place sugar and 1/4 cup water in a saucepan. Stir over low heat for 3 minutes or until sugar dissolves. Cook over high heat for 6 minutes, without stirring, or until golden brown. Remove from heat. Carefully stir in butter and cream. Stir over low heat until caramel is smooth. Boil for 1 minute. Chill for 1 hour or until thick.
To make the mascarpone cream, beat cream using an electric beater in a large bowl until soft peaks form. Add icing sugar and mascarpone and beat until just stiff. Do not over beat or mascarpone will split. Cover and chill until needed.
Trim top of cake and cut in half through the middle. Drizzle base layer with 1/4 cup caramel. Spread with 11/2 cups mascarpone cream. Position other cake layer, cut side down, on top. Spread top with remaining mascarpone cream and drizzle with remaining caramel. To serve, scatter with hazelnuts.