Make the Filling: Stir together blueberries, granulated sugar and cornstarch in a large bowl. (Add ground cinnamon if using.) Pour berry mixture into the prepared pan. Set aside.
Make the Topping: Whisk together white rice flour, tapioca starch, cornstarch, granulated sugar, baking powder, and salt. Add the butter. Using your fingers or a pastry cutter, cut the butter into the dry ingredients until no large pieces of butter remain.
Stir in 3/4 cup heavy cream until a dough forms. If the mixture is dry, add the additional tablespoon of heavy cream. Dough should form a dough ball.
Pinch the dough into small balls. Place on top of berries, leaving a little space between the biscuits. (This recipe makes a generous amount of biscuit topping. There won't be too much room between the biscuit pieces.)
Remove from the oven and place on a wire rack to cool. Serve warm with ice cream, if desired.