Prepare Bob's Red Mill Gluten Free Brownie Mix according to package instructions then let cool for one hour.
To prepare the Mocha Frosting, melt butter in a small sauce pan over medium-low heat. Pour melted butter into the bowl of a stand mixer.
Add cocoa powder to the melted butter in the bowl of the stand mixer and beat on low speed until evenly incorporated.
Measure boiling water in a liquid glass measuring cup then add espresso powder. Whisk with a fork until granules evaporate; set aside.
Rotating between espresso mixture and powdered sugar, slowly add a little of each to the mixer with the speed increased to medium. Continue alternating between espresso and powdered sugar until frosting is flight and fluffy. This should take 2-3 minutes.
Add vanilla and beat for 60 more seconds until evenly incorporated.
Spread frosting on brownies then chill 30 minutes before slicing to serve.