Preheat oven to 350°. Spray 18 muffin cups with baking spray with flour.
In a medium microwave-safe bowl, heat chocolate and butter on high in 30-second intervals, stirring between each, until mixture is melted and smooth (about 2 minutes total). Whisk in sugars until well combined. Let cool for 5 minutes. Whisk in eggs and vanilla until well combined.
In a small bowl, whisk together flour, cocoa, baking powder, and salt. Gently fold flour mixture into chocolate mixture just until combined. Spoon batter into muffin cups, filling three-fourths full.
Bake until edges are set and a wooden pick inserted in center comes out clean, 30 to 35 minutes. Let cool completely in pans.
Spread Buttermilk Chocolate Frosting onto brownies. Cover and refrigerate for up to 3 days.
Recipe by Paula Deen Magazine at https://www.pauladeenmagazine.com/frosted-cake-brownies-recipe/