In a large bowl using a handheld or stand mixer fitted with a paddle attachment, beat the butter, oil, granulated sugar, and brown sugar together on medium-high speed until fluffy and light in color, about 3 minutes. (Here’s a helpful tutorial if you need guidance on how to cream butter and sugar.) Add the egg and vanilla extract, and then beat on high speed until combined. Scrape down the sides and bottom of the bowl as needed.

In a separate bowl, whisk the flour, cocoa powder, baking soda, and salt together until combined. With the mixer running on low speed, slowly pour into the wet ingredients. Beat on low speed until combined. The cookie dough will be thick and sticky. Cover dough tightly and chill in the refrigerator for at least 2 hours and up to 3 days. Chilling is mandatory for this sticky cookie dough.

Preheat oven to 350°F (177°C). Line large baking sheets with parchment paper or silicone baking mats. Set aside.

Remove cookie dough from the refrigerator. (If the cookie dough chilled longer than 3 hours, let it sit at room temperature for about 20 minutes. This makes the chilled cookie dough easier to scoop and roll.) Scoop dough, 1 heaping Tablespoon (about 28–30g) of dough each, and roll into balls, and arrange 2–3 inches apart on the baking sheets.

Bake the cookies for 12–13 minutes or until the edges appear set. Tip: If they aren’t really spreading by minute 10, remove them from the oven and lightly bang the baking sheet on the counter 2–3x. This helps initiate that spread. Return to the oven to continue baking.

Cool cookies for 10 minutes on the baking sheet, then transfer cookies to a cooling rack to cool completely.

With a handheld or stand mixer fitted with a paddle or whisk attachment, beat the butter on medium speed until creamy, about 2 minutes. Add confectioners’ sugar, cocoa powder, heavy cream/milk, vanilla extract, and salt. Beat on low speed for 30 seconds, then increase to high speed and beat for 1 full minute. Taste. Beat in another pinch of salt if desired.

Spread the frosting on the cooled cookies with a knife or an offset spatula. Top evenly with sprinkles.

Cover and store frosted cookies at room temperature for up to 1 day, or in the refrigerator for up to 5 days. Frosting “soft-sets” on the cookies after a few hours, meaning it will slightly dry on top.