3 cups (about 600 g) fresh or frozen blueberries
16 ounces gluten free vanilla cake mix
9 tablespoons (126 g) unsalted butter, chilled
1/2 cup (4 fluid ounces) buttermilk (or milk), chilled
Whipped cream, for serving
Preheat your oven to 325°F. Grease a 9-inch x 12-inch (or 9-inch square) baking dish, scatter the blueberries in an even layer in the bottom, and set the dish aside.
Place the cake mix in a large bowl. Using a standard grater, grate the chilled butter into the cake mix and, using your clean hands, toss to coat the butter in the cake mix. Break up the lumps that form in the grated butter until the mixture is uniform and looks like very coarse sand. Scatter the butter and cake mix mixture on top of the blueberries, and stir gently to mix. Pour the chilled buttermilk evenly over the top.
Place the baking dish in the center of the preheated oven and bake until the cake is puffed and lightly golden brown on top (about 35 minutes for the 9-inch x 12-inch pan, 40+ minutes for the 9-inch square pan). Do not overbake or the bottom will burn. Remove from the oven and allow to cool in the pan for 10 minutes before serving with a dollop of whipped cream.