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.

In the bowl of an electric mixer, beat butter until smooth. Gradually add in powdered sugar about 1/2 cup at a time with the mixer on low, increasing it to medium-high speed once all the sugar has been added. Add the cocoa, vanilla and hot fudge, and mix until completely combined (while scraping down the bowl every few minutes) for about 5-6 minutes. If the frosting seems very thick, simply add some milk 1 tablespoon at a time and mixer for another minute or 2 until it comes together.

Note: this makes enough frosting for me to frost 12 cupcakes with a pastry bag. If you use a spatula for frosting, you will most likely have frosting left over. If you use a pastry bag and add a lot of frosting, you may not have enough. I add about 1/4 cup of frosting on my cupcakes when piping. If you have some leftover, simply store it in an air-tight container in the fridge (a few days) or the freezer (3-4 months!).