Preheat your oven to 350°F (175°C). Wash and cut rhubarb into 1-inch pieces. Grease a 9-inch round baking pan with butter. Arrange the rhubarb pieces in a colorful pattern in the bottom of the pan.

Melt 4 tablespoons of butter in the greased pan over medium heat. Sprinkle ¾ cup of brown sugar over the melted butter and stir to combine. Cook for 2-3 minutes until dissolved and caramelized. Pour over rhubarb.

In a mixing bowl, cream together ½ cup of sugar and ½ cup of room-temperature butter with a hand mixer until light and fluffy. Add 2 eggs one at a time. Pour in ½ cup of milk and blend. In another bowl, mix 1 cup flour, 1 teaspoon baking powder, and a pinch of salt. Gradually add dry to wet ingredients, stirring until combined.

Gently pour the cake batter over the caramelized rhubarb. Spread evenly without disturbing the rhubarb layer.

Bake for 30-35 minutes. Check doneness with a toothpick—should come out clean. The top should be golden brown.

Let the cake cool in the pan for 10 minutes. Run a knife around the edges, then invert onto a large plate to release the cake.

Allow the cake to cool completely before slicing and serving.