We use a Betty Crocker cake mix for this recipe. You can use another brand and just replace the amount of water called for on the back of the box with grapefruit juice. In this case the amount of water called for on the back of the Betty Crocker cake mix box was 1 cup.
3 tablespoons unsalted butter, cut into 24 small pieces
3/4 cup lightly packed brown sugar
2 cups fresh blueberries or frozen blueberries that have been thawed and drained
Juice and grated peel of 1 medium grapefruit
1 box Betty Crocker® French vanilla cake mix
Vegetable oil and eggs called for on back of cake mix box
1 cup whipped cream
1 tablespoon ground cinnamon
Heat oven to 350 degrees F (or 325 degrees F for a dark or nonstick pan).
Place 1 piece of butter in each of 24 muffin cups then place the pan into the oven until the butter has melted. Divide the brown sugar and blueberries between muffin cups.
Add grapefruit juice to a measuring jug that holds at least 1 cup. Fill the jug to the 1-cup line with water. (One grapefruit should create about 1 cup of juice. If you have too much, only use 1 cup. If you do not have enough, add water until you reach 1 cup).
Add the cake mix, oil, eggs, grapefruit juice and grapefruit zest to a large bowl. Mix with a mixer on medium speed or beat vigorously by hand for 2 minutes. Pour the batter over blueberries, filling each cup three-fourths full.
Bake until a toothpick inserted in center comes out clean, 17 to 22 minutes (20 to 25 minutes for dark or nonstick pan). Immediately run small knife around the sides of each cake to loosen.
Cool in the pan, on a cooling rack for 15 minutes. Run a knife around the sides of cakes for a second time. Place a heatproof serving plate upside down over the pan. Turn the plate and pan over and set down for 5 minutes. Remove pan and cool 30 minutes.
Serve the mini cakes with a dollop of whipped cream and sprinkle of cinnamon on top. Store covered in refrigerator.