Sliced Blueberry Rolls

Dough:
225ml whole milk
75g unsalted butter
300g all purpose flour
125g whole wheat flour
70g granulated sugar
finely grated zest of 1 lemon
pinch of salt
2 ½ teaspoons dried yeast
1 egg
1 teaspoon vanilla extract

Filling:
75g unsalted butter, softened
75g granulated sugar + about 2 teaspoons extra, for sprinkling over the rolls
1 teaspoon ground cinnamon
200g fresh or frozen (unthawed) blueberries

Dough: in a small saucepan, heat milk and butter together until milk almost boils and butter melts almost completely. Cool to lukewarm.
In the large bowl of an electric mixer, combine the flours, sugar, zest, salt and yeast. Make a well in the center, add the egg, vanilla and the milk mixture and, using the dough hook, mix in medium-low speed until a smooth dough forms, about 5 minutes. Transfer to a lightly buttered large bowl, cover with plastic wrap and prove for about 1 hour.
Filling: with an electric mixer, cream butter, sugar and cinnamon together until smooth. Set aside.
Lightly butter a 20x30cm (12x8in) baking pan. Roll out the dough on a lightly floured surface until you get a 30x50cm (12x20in) rectangle. Spread the filling evenly over the dough then sprinkle with the blueberries. Roll it like a cylinder, beginning with the longer side. Cut into 8 slices then arrange them about 2.5cm (1in) apart onto the prepared pan. Cover with a clean kitchen towel and leave to prove for about 1 ½ hours (if you’re using frozen berries prove your rolls for 2 hours); in the meantime, preheat the oven to 190°C/375°F.
Sprinkle the rolls with the extra granulated sugar and bake for about 20 minutes or until risen and golden brown. 

Makes 8