01 -
Set oven temperature to 350°F and prepare a 9x13 inch baking pan by greasing or lining with parchment paper.
02 -
In a large bowl, whisk together melted unsalted butter and granulated sugar until fully blended; add eggs one at a time, whisking after each addition, then incorporate vanilla extract.
03 -
Sift together flour, unsweetened cocoa powder, salt, and baking powder in a separate bowl.
04 -
Gradually fold the dry mixture into the wet ingredients until nearly combined; stir in cooled strong brewed coffee and instant coffee granules gently to achieve a thick, glossy batter without overmixing.
05 -
Pour batter evenly into prepared pan, smooth the top, and bake for 28 to 32 minutes until the center is just set yet still fudgy; allow to cool completely in the pan.
06 -
Whisk powdered sugar, whole milk, and vanilla extract in a medium bowl until smooth; add green gel food coloring a few drops at a time until desired vibrancy is reached, maintaining a thick yet pourable consistency by adjusting milk as needed.
07 -
Cut cooled brownies into squares and generously drizzle each piece with the green slime glaze just before serving for a vivid, spooky effect.