• Preheat oven to 350 degrees F. Line a 9 x 13 pan with foil, leaving an overhang on the ends for easy removal.
• In the bowl of a stand mixer, combine the butter and sugar until smooth.
• Add all four eggs, and mix until fluffy and well-combined.
• Add the salt, cocoa, and vanilla, mix until combined.
• Add the flour and mix on low speed until mixture is combined. It will be thick.
• Push batter into prepared pan and bake for 25 to 30 minutes, until a toothpick inserted in the center comes out clean.
• Remove from oven and set aside to cool before frosting.

To make the frosting:

• In a large bowl using a stand or hand-held electric mixer beat the butter for about 1 minute to soften. Turn the mixer down to low speed and beat in 2.5 cups of powdered sugar. Once the powdered sugar begins to incorporate, turn the mixer up to medium speed and continue beating. Then mix in the coffee and whipping cream. Add in more powdered sugar to get the desired texture and sweetness you prefer. You can always add more cream, 1 tablespoon at a time, if you need to thin it out some.
• Remove the cooled brownies from the pan and spread the coffee buttercream on as evenly as possible. Cut into squares and enjoy.