Design Eat Repeat

To Make Brownies:
14 Before you start, be sure to read through the tips & tricks in the post above for ingredient notes, step-by-step photos, and FAQ's!
13 Preheat oven to 350° F.
0 In a large microwave-safe bowl, add unsalted butter (¾ cup, i.e. 1 ½ sticks), granulated sugar (1 ½ cups), and light brown sugar (½ cup). Microwave for 2 minutes, stir, then micowave for 30 more seconds. Carefully remove bowl from microwave and stir until smooth. Let cool for 2-3 minutes.
2 To the butter and sugar mixture, stir in the 3 whole eggs, 1 extra egg yolk, vanilla extract (1 tablespoon), salt (1 teaspoon), baking soda (¼ teaspoon), and cornstarch (2 tablespoons). Once combined, mix in cocoa powder (1 ½ cups) and all-purpose flour (1 cup) until just combined (I recommend mixing by hand to prevent overmixing). Stir in vegetable oil (¼ cup).
3 Spray a light metal 9x13 pan with nonstick cooking spray, then press on a piece of parchment paper to cover the bottom and long sides to create a parchment sling (photo in faq section above!). This will help you lift the brownies out of the pan so that you can cut them nicely. Spread brownie batter into the prepared pan until smooth.Note: baking these in glass or dark metal pans may cause them to overbake and become bitter. If that's the only pan you have, lining with parchment on all sides becomes more crucial. You will also likely need to bake them for less time to prevent overbaking.
4 Bake at 350° F for 24-26 minutes or until center looks set and no longer jiggles when you gently shake the pan (do not overbake). Let cool completely in pan (once the pan is no longer hot to the touch, you can speed up cooling by placing pan in refrigerator.
To Make White Chocolate Ganache:
11 In a medium-sized, heat-safe bowl, place white chocolate chips (2 cups; i.e. 11 or 12oz bag).
7 In a separate medium-sized bowl or large liquid measuring cup, heat heavy cream (⅓ cup + 2 tablespoons) in the microwave for 1 ½ minutes, or until it begins to bubble (it’s important that your measuring cup or bowl is at least 4x taller than the milk line, as it will expand when it’s heating and can easily overflow). You can also heat this on the stove; remove it once it begins to steam.
8 Use a hot pad to carefully remove milk from the microwave and pour over the white chocolate chips (do not stir yet!). Use a fork to press down the chips so that they're fully covered with the cream.Allow to sit for about 1 minute, then use a fork to stir; it will be thick and will likely look like it's going to mix together but keep at it for at least 30 seconds. At this point, the mixture will likely be thick and look a little grainy. Microwave in 15-second increments on 50% power, stirring in between each until smooth and pourable (I did four rounds of this). Stir in green food coloring, then pour over cooled brownies and spread until smooth. Tap the pan on the counter to help smooth out any bumps.
10 Immediately top with rainbow chip sprinkles (or any other sprinkles you have!). Place in refrigerator for 1-2 hours to help the ganache harden (you can speed this up by putting in the freezer!).Use a large knife to slice bars into trees, as shown in the photo instructions in the blog post above. I highly recommend that your bars are chilled before slicing to ensure a clean cut.
12 Storage: I recommend refrigerate the frosted bars, as they get even better & fudgier on Day 2! You can also keep them at room temperature for up to 2 days, if desired.