For the Topping:

•4 tablespoons unsalted butter

•1/2 cup packed brown sugar

•3 cups fresh rhubarb, cut into 1/2-inch pieces

•1 tablespoon vanilla extract

For the Cake:

•1 1/3 cups all-purpose flour

•1 cup granulated sugar

•1/3 cup unsalted butter, softened

•1 1/2 teaspoons baking powder

•1/2 teaspoon salt

•2/3 cup whole milk

•1 large egg

•1 teaspoon vanilla extract

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Prepare the pan: Preheat oven to 350°F (175°C). Place 4 tablespoons of butter in a 9-inch round cake pan or cast-iron skillet. Put the pan in the oven for 3-4 minutes until butter melts.
Create the topping: Carefully remove the hot pan and sprinkle brown sugar evenly over the melted butter. Arrange rhubarb pieces in a single layer over the brown sugar. Drizzle with vanilla extract.
Make the cake batter: In a large bowl, combine flour, granulated sugar, softened butter, baking powder, salt, milk, egg, and vanilla extract. Beat with an electric mixer on low speed for 30 seconds, then on high speed for 3 minutes.
Carefully spoon the batter over the rhubarb, spreading gently to cover without disturbing the fruit arrangement.
Bake: Place in preheated oven and bake for 40-45 minutes, or until a toothpick inserted in the cake portion comes out clean.
Cool and flip: Let cool in pan for 5 minutes. Run a knife around the edges to loosen. Place a serving plate upside down over the pan, then quickly flip both together. Let the pan rest for 1 minute before lifting off.

Serve: Serve warm or at room temperature. Store covered at room temperature for up to 3 days.