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 microwave 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 eggs (3 large), egg yolk (1 large), 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 from 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 post 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 Easy Chocolate Ganache
6 Before you begin: Note that the ganache will not completley "harden" as a traditional ganache with heavy cream would. See notes section below for tips & tricks, as well as an alternate recipe using heavy cream.
11 In a medium-sized bowl, place semi-sweet chocolate chips (1 ½ cups) and chopped unsalted butter (6 tablespoons).
7 In a separate medium-sized bowl or large liquid measuring cup, heat milk (½ cup) in the microwave for 1 ½ - 2 minutes, or until it begins to bubble (it’s important that your measuring cup or bowl is at least 3x 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 chocolate chips and butter (do not stir yet!). Allow to sit for about 3 minutes, then use a whisk to stir until smooth and creamy. It may take a minute or two for it to come together; it often looks separated at first. Allow to cool for about 30 minutes before pouring over cooled brownies.
To Decorate:
10 Once brownies are cooled, re-stir the ganache and spread over brownies. Immediately top with rainbow chip sprinkles (or any other sprinkles you have!). Place in refrigerator for 1-2 hours to help the frosting harden (you can speed this up by putting in the freezer!).Use a large knife to slice bars ( I highly recommend that your bars are chilled or frozen before slicing to ensure a clean cut).
12 Storage: Refrigerate the frosted bars. You can also keep at room temperature for up to 1 day (just note the frosting will soften). They get even better & fudgier on Day 2!