Preheat your oven to 180°C (350°F).
Butter a 23 cm (9-inch) or 25 cm (10-inch) round cake pan. Press a sheet of wide parchment paper, that is large enough to come up the sides of the baking pan, smoothing out the bottom so it sticks and creasing the sides. Lightly brush the parchment with butter.
Coat blueberries with flour.
Place 1 sheet of phyllo dough on a clean surface and brush it with melted butter. Then arrange blueberries on the bottom of the phyllo dough sheet.
Roll the dough so you enclose the berries within the dough, and continue to roll it until you get to the half of the phyllo sheet. Using your fingers wrinkle up the rest of phyllo dough, so you get that ruffled look. Wind it up into a loose, messy spiral and place it into a prepared pan. (See photos in the post for a step-by-step tutorial)
Repeat with the rest of the phyllo dough and blueberries until your pan is filled.
Brush the tops of the rolls with the remaining butter. Transfer to the oven and bake 20-25 minutes, until golden brown.
Take it out from the oven, and let it rest for 10 minutes, while you're making the custard.
For the custard, heat milk, heavy cream, and vanilla in a small saucepan, until it starts to boil.
In a bowl, beat eggs with brown sugar. Pour some of the milk mixture, and whisk quickly. We must temper the eggs to prevent them from curdling. Continue pouring the milk mixture little by little at the same time that we whisk until it is completely incorporated.
Pour the custard over the rolls, sprinkle some blueberries on top, and continue to bake until golden and custard is set, around 20-25 minutes.
Let cool for 15 minutes, then lift from the pan and serve dusted with cinnamon and powdered sugar.