Place an oven rack in the center of the oven, then preheat to 375°F. In a bowl, combine the flour, biscuit mix, baking powder, vanilla powder, and a pinch of salt. Whisk to combine.

Working in 30 second increments, melt the butter in a large bowl in the microwave (or melt in a small pot on the stove, then transfer to a large bowl). Add the sour cream, egg, and sugar; whisk to thoroughly combine.

Wash and dry the blueberries. Add the dry ingredients to the bowl of wet ingredients; using a spatula, gently mix until no white streaks remain. Gently fold in the blueberries until thoroughly combined.

Lightly grease the rounds of a cupcake tin (or line with cupcake liners). Transfer the batter to the prepared rounds. Bake 16 to 20 minutes, or until the batter is cooked through and a toothpick inserted into the middle comes out clean. Remove from the oven and let cool completely (for quicker cooling, place in the refrigerator).

While the cupcakes cool, in a bowl, vigorously whisk together the lemon curd and mascarpone until smooth. Divide the frosting among the cooled cupcakes and spread into an even layer. Enjoy!