8 tablespoons (½ cup) unsalted butter, room temperature and divided
½ cup dark brown sugar
2 cups blueberries, fresh or frozen
2 (7 oz.) packages Martha White Blueberry Muffin Mix
¾ cup whole milk
1 teaspoon vanilla extract
Whipped cream, optional
Heat oven to 325ºF.
Place a 10-inch cast iron or other ovenproof skillet over medium heat. Add ¼ cup butter to the skillet and melt. Add the brown sugar and stir until the mixture is thick and caramelized, about 1 minute. Remove from heat. Add the blueberries and arrange in an even layer across the bottom of the pan.
In a medium bowl, stir together the muffin mix and remaining ¼ cup butter until crumbly and well-combined. Add the milk and vanilla and stir just until the batter comes together.
Spoon batter into the prepared pan so that it sits atop the blueberries in an even layer.
Transfer the skillet to the oven and bake until the cake is risen and golden, 35 to 40 minutes.
Let cool for 5 minutes, then use a butter knife to loosen the edges of the cake. Place a large plate over the skillet, quickly and carefully invert the pan so that the cake releases and rests on the plate with the blueberries facing up. Slice and serve warm with whipped cream, if desired.