Step 1

Preheat oven to 350°F.

Step 2

In a large bowl, use an electric mixer on medium-high speed to cream softened unsalted butter (½ cup, 1 stick), light brown sugar (½ cup), and granulated sugar (¼ cup) for 2 minutes until light and fluffy.

Step 3

Add the egg (1 large), vanilla extract (1 teaspoon), and light corn syrup (2 tablespoons) and mix on low speed until combined. Scrape down the sides of the bowl so everything can combine.

Step 4

Add in baking soda (¼ teaspoon) and salt (¼ teaspoon) and mix for another 5-10 seconds until combined.

Step 5

Lastly, add dark cocoa powder (½ cup) and all-purpose flour (1 ¼ cups) and mix on medium until combined.

Step 6

Scoop dough into 1/4 cup (4 tablespoons) sized scoops (I use this stainless steel cookie scoop!), roll into balls, then gently flatten each dough ball into a 1/2" thick disc; this will help them spread instead of be puffy.

Step 7

Bake at 350°F for 11-12 minutes. Do not overbake. The centers may look a tiny bit wet but will continue to bake on the hot pan, allowing them to firm up without overbaking.Allow to fully cool before decorating.

Step 8

In a medium-sized, heat-safe bowl (cereal bowl sized), place semi-sweet chocolate chips (¾ cup).

Step 9

In a separate medium-sized bowl or large liquid measuring cup, heat heavy cream (¼ cup) in the microwave for 30-45 seconds, 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.

Step 10

Use a hot pad to carefully remove milk from the microwave and pour over the 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 3 minutes, then use a fork to stir; it will be thick and will likely look like it's not going to mix together but keep at it until smooth & creamy.If your mixture doesn't melt down, microwave in 10 second increments at 50% power, stirring in between each increment until smooth.

Step 11

Spoon 1 tablespoon of chocolate ganache on each cookie and top with rainbow chip sprinkles. Place in refrigerator for 30-60 minutes to help the ganache harden.

Step 12

I recommend refrigerate the cookies as they get even better & fudgier when chilled! You can also keep them at room temperature for up to 2 days, if desired.