To prepare the crumble

1. Add the diced butter to a small saucepan or skillet set over medium heat. Stir with a whisk or swirl the pan occasionally to ensure the butter is melting evenly. Once melted, the butter will sizzle, foam, and eventually start forming little golden bits on the bottom of the pan. Continue cooking and stirring regularly until the butter has taken on an amber color and nutty aroma. Take care not to burn the butter. Remove the pan from heat and pour the brown butter into a medium sized mixing bowl.
2. To the mixing bowl, add the brown sugar, salt, and vanilla bean paste, and stir to combine. Add the flour and fold until large crumble clumps form and the flour is well incorporated. Set aside in the fridge while you prepare the cake.

To prepare the cake

1. Preheat the oven to 375 degrees. Spray a 9” square or round baking pan with cooking spray and line the bottom with a square of parchment paper for easy removal, if desired.
2. Combine the flour, baking powder, and salt in a small bowl and set aside.
3. In the bowl of a stand mixer or a large mixing bowl, cream the butter and sugar together on medium speed until light and fluffy, about two minutes. Add the egg, vanilla bean paste, and milk and beat on low until well combined. Scrape the bowl, if needed, to ensure that everything is evenly incorporated. Add about 1/3 of the dry ingredients and beat on low speed to combine. Add ½ of the sour cream and beat to combine. Repeat this process once more and then add the remaining dry ingredients, stirring on low to combine. Scrape the sides of the bowl and fold in any unincorporated bits. Your batter will be quite thick.
4. Spread the batter evenly in the bottom of your lined pan. In a separate bowl, toss the rhubarb and raspberries together with the brown sugar and spread this mixture on top of the cake batter. Gently press the fruit into the batter a bit. Crumble the brown butter streusel evenly on top of the fruit and place the pan in the preheated oven. Bake for about 50 minutes, or until a cake tester inserted comes out clean (See notes). Allow to cool slightly before eating. Cake will keep best at room temperature, covered tightly in plastic wrap.