2) Brush 1 tbsp of the butter over the dough and then sprinkle the nut mixture on top, leaving a 1cm border along the top and bottom sides.
3) Starting at one long side, roll up the dough jam-roll-style, forming a log. Pinch the seam to seal. Cut the log into 9 equal pieces. Arrange the rolls, cut side down, in the prepared baking dish, spacing evenly.
4) Cover the dish with cling film. Let the rolls rise in a warm draft-free area until puffed, about 45 minutes. Meanwhile, position the rack in the centre of the oven and preheat the oven to 170C/Gas 3.
5) Bake the cinnamon rolls uncovered until the tops are golden brown, about 25 minutes. Meanwhile, whisk the icing sugar, mascarpone cheese and buttermilk in a medium bowl until smooth and creamy.
6) Brush the remaining 1 tbsp of butter over the baked cinnamon rolls and then drizzle the cheese mixture over the top. Serve warm.