For the Brownies
Preheat oven to 325°. Line a 9x9 baking pan with parchment paper, overlapping each side - this makes it much easier to pull out to cool and cut. Set aside.
Place butter, cocoa and sugar in a microwave-safe bowl and microwave for about 1 minute and 30 seconds, or until butter is melted. Mix everything together until smooth. Batter will be very gritty but will smooth out once eggs are added. Add in eggs and vanilla and mix until smooth. Pour in the flour and mix until just combined. Then, mix some more until smooth and somewhat glossy. The batter will be very thick.
Pour the batter into your prepared baking pan and smooth it out until level. Bake for 25-30 minutes or until a toothpick comes out slightly moist with a little bit of batter left on it - this is what makes them fudgy!! Let them cool in pan for 5 minutes then pull out using the parchment paper and set on wire rack or counter to finish cooling completely. While brownies are baking, start preparing your frosting.
For the Frosting
Cream together butter, cream cheese and peanut butter until combined. Add in the salt and powdered sugar and mix until completely smooth. Once brownies are completely cooled you can frost them. If using, add sprinkles on top. Cut them into squares with a wet knife and serve!