In a stand mixer with a paddle attachment, or large bowl with a hand mixer, add your butter and both sugars. Combine at a medium speed for about a minute. Scrape down the sides of your bowl.

Add in your egg and vanilla extract. Continue to mix for about two minutes until your mixture is lightened. Scrape down your bowl again and your mixer attachment. Mix again for another minute.

Next, add in your cocoa powder and mix for a minute until it's completely incorporated. Scrape down the sides of your bowl and mixer attachment. Then, add in the rest of your dry ingredients - flour, baking soda, baking powder and salt. Mix at a low speed until your dough just forms. Pause and finish mixing by hand with your spatula, being sure to scrape down to the bottom of your bowl.

Measure out your sprinkles in a medium bowl and set aside. Scoop your cookie dough onto a parchment lined cookie sheet. It should make about 10 scoops, each 2 ½ to 3 tablespoons, or about 50g per scoop. Roll each scoop in your palms to smooth out the edges and then roll each to coat in sprinkles. Cover your cookie sheet in plastic wrap and place in the fridge to chill up to 12 hours, or at minimum 3.

In a bowl, combine your butter, vanilla, powdered sugar, cocoa powder and salt. Use a spatula to mix until completely smooth. It will seem crumbly at first but keep mixing and it will dampen eventually. Once mixed completely, add in one tablespoon of heavy cream and mix until combined. Then switch to a mixer - you can use a hand mixer, or stand if you prefer. Mix for a minute or two at a medium to high speed until your frosting increases in volume and lightens in color. Add another tablespoon of heavy cream and continue to mix for another two minutes. If your frosting is too thick, add another tablespoon of heavy cream and mix one final time.

Remove your chilled frosting from the fridge so it can come to room temperature in time to frost your cookies. Preheat your oven to 350 degrees. Bake your cookies for 11-13 minutes and then let them cool completely, about an hour.

Once your frosting has come to room temperature, and your baked cookies have cooled completely, use a spatula to mix your frosting to ensure it's smooth throughout and softened. Add about 1 ½ tablespoons of frosting onto each cookie, spread it however you'd like and enjoy!

The frosting on your cookies will harden once you leave your cookies out at room temperature, and then you can store them in an airtight container in the fridge for about 5 days. They can be enjoyed at room temperature or straight from the fridge.