Ingredients

• ½ cup (8 tablespoons) unsalted butter, room temperature
• 2 drops lemon oil
• 1/2 cup sugar
• 1/4 tsp. stevia drops
• 1 egg, room temperature
• 1 tsp. vanilla
• 1/2 cup white rice flour
• 1/2 cup arrowroot flour
• 1/2 cup almond flour
• 1/4 cup coconut flour
• 2 tsp. baking powder
• 1 tsp. kosher salt
• 2 cups fresh blueberries
• ½ cup buttermilk

1. Preheat the oven to 350ºF. Using a stand mixer or hand-held mixer, cream the butter with the lemon zest and the 1 cup minus 1 tablespoon of sugar until light and fluffy.
2. Add the egg and vanilla and beat until combined. Meanwhile, toss the blueberries with ¼ cup of flour, then whisk together the remaining flour, baking powder and salt.
3. Add half of the flour mixture to the batter, and stir with spatula to incorporate. Add all of the buttermilk. Stir. Add remaining flour, and stir until flour is absorbed. Fold in the blueberries. (Leave excess flour from the blueberry bowl behind.) 
4. Grease an 8- or 9-inch square baking pan with butter or coat with non-stick spray. Spread the batter into the pan. Sprinkle the batter with the remaining tablespoon of sugar. Bake for 35 to 45 minutes — a 9-inch pan will be done in closer to 35 minutes; an 8-inch pan usually needs 40 to 45 minutes. Check with a toothpick for doneness.