Rhubarb Upside-down Cake

Preheat oven to 350°F (175°C).

Melt 1 stick of butter in a 9-inch cast-iron skillet or ovenproof baking pan over medium heat, then sprinkle brown sugar evenly across the bottom.

Arrange the cut rhubarb pieces evenly over the brown sugar and butter mixture.

In a small bowl, whisk together the flour, baking powder, salt, and cinnamon until combined.

In another bowl, cream together the remaining butter and granulated sugar until light and fluffy, then beat in the egg and add the vanilla extract.

Alternate adding the flour mixture and milk to the wet ingredients, stirring until just combined to form a thick batter.

Spread the batter evenly over the rhubarb layer in the skillet.

Bake in the preheated oven for 40-45 minutes until a toothpick inserted into the cake (not the fruit) comes out clean and the top is golden brown.

Remove from the oven and let cool for 5 minutes, then run a thin knife around the edges to loosen the cake.

Place a serving platter over the skillet and carefully invert the cake onto the platter so the caramelized rhubarb is on top.

Serve warm, optionally with whipped cream or vanilla ice cream.