Start by combining your butter and chocolate chips in a large microwave-safe bowl. Microwave in short 15 to 30 second bursts, stirring well between each one, until the mixture is completely smooth and glossy.
Stir the cocoa powder and instant coffee into your melted mixture until fully combined. Add both sugars next and stir again until the batter looks thick, shiny, and evenly blended.
Beat in your eggs and egg yolk one at a time, then stir in the vanilla. In a separate bowl, whisk the flour, baking powder, and salt together before folding the dry mix into your wet batter until just combined.
Scoop the batter into a greased mini muffin tin and bake at 350°F for about 18 minutes. As soon as they come out of the oven, press a rounded teaspoon into the center of each brownie to create a small well for the frosting.
While the brownies cool completely, beat your softened butter and cream cheese together until smooth. Mix in the vanilla, then gradually add the powdered sugar until the frosting is thick and pipeable, splitting it into bowls to tint with your gel colors.
Once your brownie bites are fully cooled, spoon or pipe the tinted frosting into each well so it looks like it's bubbling over the edge. Finish with sprinkles and a candy eyeball or two before serving.