Preheat the oven to 450°F. Generously grease a 9 x 13-inch baking dish with butter.

Fluff the flour, spoon it into measuring cups and level it. In a large bowl, whisk together the flour, sugar, baking powder, baking soda and salt.

Add the cubed butter and shortening. Use a pastry cutter until the pieces are small and the mixture resembles oatmeal.

Gently mix in the buttermilk and vanilla until the dough comes together. Add the blueberries and gently fold them in without popping them.

On a floured work surface, gently press the dough into a disc about 2 inches thick. Cut out 24 biscuits with a 2-inch cutter and place them in the prepared dish. Or cut the dough into 24 2-inch squares with a sharp knife.

Put the dish in the oven and immediately reduce the temperature to 425°F. Bake for 30 to 40 minutes. At 30 minutes, insert a toothpick deep into the center of a biscuit. It should come out clean. If needed, bake for another 5 to 10 minutes. Loosely cover with foil if the biscuits darken before they are done.

Let the biscuits cool in the pan. When they are mostly cool, whisk the powdered sugar, lemon juice and heavy cream until smooth, then drizzle the glaze over them.