Preheat oven to 350 degrees F. Grease an 8 or 9 inch baking dish, square, round, or oval, and set aside.

Combine 2 Tbsp of the white sugar with the raspberries and rhubarb. Toss with your hands; set aside. Melt the butter and let cool (but not resolidify).

In a mixing bowl, combine the flour, baking powder, baking soda, and salt. Whisk to combine.

In a separate bowl, whisk the white sugar, melted butter, and egg, egg yolk, and vanilla until sugar begins to dissolve and become glossy and thick. Add the buttermilk.

Incorporate the flour mixture into the wet ingredients, adding about 1/3 of the dry ingredients at a time, until a smooth batter forms—don’t overmix. Pour into the prepared pan, and smooth the top using a spatula. Spread the sugared fruit over the top evenly, any leftover juices included. Then place in the oven for 40-45 minutes, or until a cake tester inserted into the center of the cake comes out clean, the center is no longer jiggly, and the edges are a golden brown and beginning to pull away.

Allow cake to cool slightly before serving with whipped cream, vanilla ice cream, or a sweetened dollop of crème fraîche!