Preheat oven to 350 degrees F.
Spread blueberries on a baking sheet (with sides) and roast for 10-15 minutes, or until berries start to burst. Remove from oven and let cool.
Beat butter and sugar together in the bowl of an electric mixer until fluffy, about 4-5 minutes. Add eggs and vanilla extract and mix until combined, another 2-3 minutes. Add flour, baking powder and salt with the mixer on low speed, then gradually increase speed. Scrape in blueberries and beat on medium speed until the berries break down – about 2-3 minutes. At this point, the berries most likely have enough liquid that you only need to add a small amount of milk to the batter. Begin adding 1 tablespoon of milk at a time and beat with the mixer until the batter is smooth. You definitely want it to be cake batter and not dough.
Pour into cupcake liners, filling 2/3 of the way full, and bake for 18-22 minutes or until cake is set. Let cool completely, then frost.