Preheat oven to 180°C/350°F/Gas 4. Spray a half-sheet baking pan (approximately 12" x 19") with baking spray then cover with a sheet of parchment paper.
Place the butter, sugars, eggs, cooled coffee, and vanilla into a large bowl and whisk until combined and fluffy.
Sift or whisk the cocoa powder, espresso grounds, flour, cornstarch, and baking powder in a medium bowl to combine.
Gradually beat the dry ingredients into the wet ingredients until just combined.
Spread the brownie batter onto the cookie sheet and bake in the oven for 22-26 minutes or until a toothpick inserted into the center comes out clean.
Place on a wire rack to cool completely.
Using a 4-inch Mickey Mouse cookie cutter, cut out 12 Mickey Mouse heads and place them onto a wire rack
To make the chocolate ganache, place the heavy whipping cream and espresso grounds into a saucepan on medium heat and bring to a simmer.
Place the chocolate chips in a medium bowl and pour the hot cream mixture over. Whisk until smooth.
Gradually pour the ganache over the Mickey Mouse shaped brownies, making sure the tops and
sides are completely covered.
Sprinkle the rainbow chips on top.
Allow the ganache to harden before serving. Enjoy and have a magical day!