5tablespoonsunsalted buttercold and cut into cubes
1/2cupwhole milkplus more for brushing
2tablespoonssanding or turbinado sugar
Preheat the oven to 400°F.
In a large bowl, toss the blueberries and rhubarb with the sugar and flour until evenly coated. Place the filling in a buttered baking dish (about 2 quarts and 1 1/2 to 2 inches deep). Set aside.
Make the dough by whisking together the flour, sugar, baking powder, salt, and baking soda in a bowl. Cut in the butter using a pastry cutter or your fingers until you have coarse crumbs. This can also be done in a food processor. Add the milk and stir gently until it just forms a sticky dough; do not over mix. Place large spoonfuls of dough over the fruit in the baking dish. Lightly brush the dough with milk and sprinkle with sugar.
Bake for about 30 minutes, or until the fruit filling is bubbling and biscuits are puffed and golden. Let it sit at room temperature to cool for about 25 minutes.
Serve warm with vanilla ice cream.
If you make the cobbler ahead of time, you can reheat the dessert in a 350°F oven.