Preheat oven to 325°. Spray an 8-inch square baking pan with cooking spray. Line prepared pan with parchment paper, letting excess extend over sides of pan.
In a large bowl, whisk together cocoa powder, granulated sugar, brown sugar, coffee granules, and salt.
In a medium saucepan, melt butter over medium heat. Remove and let cool for 5 minutes. Gradually whisk butter into cocoa mixture until combined (mixture will be thick). Add vanilla extract, whisking until incorporated. Add eggs, one at a time, whisking well after each addition until well incorporated. Fold in flour just until combined. Spread batter into prepared pan.
Bake until a wooden pick inserted in center comes out with a few moist crumbs, 25 to 30 minutes. Let cool completely in pan.
Using an offset spatula, spread an even layer of Plum Buttercream over cooled brownies. Cover and refrigerate for 1 hour.
In the top of a double boiler set over simmering water, combine chopped chocolate and oil, stirring often until melted and smooth. Working quickly, pour chocolate mixture over chilled brownie. Using a small offset spatula and tilting brownie pan as needed, spread chocolate mixture in an even layer. Tap pan on counter several times to smooth chocolate mixture and release any air bubbles. Refrigerate, uncovered, for at least 1 hour.
Using parchment paper as handles, remove brownie from pan and onto a cutting board. Using a warm serrated knife in a gentle sawing motion, trim edges from brownie and cut into 25 (approximately 1½-inch) squares.
Garnish with freshly grated pistachios, if desired.
Notes
*We used Kerrygold.
Recipe by TeaTime Magazine at https://teatimemagazine.com/brownies-with-plum-buttercream-recipe/