Preheat your oven to 325°F (165°C) and prepare a 9-inch springform pan by greasing it with butter and lining the bottom with parchment paper.

In a medium saucepan over low heat, melt the butter and light brown sugar together until bubbly and smooth. Pour this caramel mixture evenly into the prepared springform pan.

In a mixing bowl, combine chopped rhubarb, raspberries, granulated sugar, and cornstarch. Toss until coated and let sit for 10-15 minutes.

In a separate bowl, whisk together the flour, ground almonds, baking powder, and salt. In a mixer, cream the remaining butter and granulated sugar until light and fluffy. Add eggs one at a time and mix in vanilla.

Gradually incorporate the dry mixture into the wet mixture, alternating with sour cream until combined, without overmixing.

Layer the fruit evenly over the caramel in the pan, then spoon the cake batter over the fruit, smoothing the top.

Bake for 1 hour and 40-50 minutes, or until a toothpick inserted comes out clean and the edges are golden brown.

Let the cake cool in the pan for 15 minutes, then run a knife around the edge and invert onto a serving plate.