Preheat your oven to 350°F (175°C) and grease a 9-inch round cake pan with unsalted butter.

In a medium mixing bowl, combine chopped rhubarb, hulled strawberries, granulated sugar, and melted butter. Spread the fruit mixture evenly across the bottom of your prepared cake pan.

In a separate bowl, whisk together all-purpose flour, baking powder, and salt.

In a large mixing bowl, beat the butter and sugar together using an electric mixer on medium speed until light and fluffy (about 3 minutes).

Gradually add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.

Alternately add the flour mixture and milk to the wet ingredients, starting and ending with flour. Mix until just combined.

Pour the batter over the prepared fruit mixture in the cake pan and smooth the top.

Bake for 40-45 minutes until a toothpick inserted comes out clean and the edges pull away from the pan.

Cool in the pan for about 10 minutes before carefully inverting onto a plate.