In a large bowl, combine the softened butter, vegetable oil, granulated sugar, and brown sugar. Beat on medium-high speed until fluffy and light in color, about 3 minutes. Add the egg and vanilla extract, and beat on high speed until well combined.

In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt. Slowly incorporate the dry ingredients into the wet mixture, beating just until combined. Cover the dough and chill in the refrigerator for at least two hours.

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

Remove chilled cookie dough. Let it sit at room temperature for 20 minutes if chilled for over three hours. Scoop heaping amounts of dough (about 28–30g each) and roll into balls. Space them 2–3 inches apart on prepared sheets.

Bake cookies for 12–13 minutes, or until the edges appear set. If they aren’t spreading by the 10-minute mark, bang the baking sheet on the counter and return to the oven.

Allow cookies to cool on the baking sheet for 10 minutes before transferring to a cooling rack.

In a separate bowl, beat the softened butter on medium speed until creamy. Add the confectioners’ sugar, cocoa powder, heavy cream or milk, vanilla extract, and a pinch of salt. Beat on low speed for 30 seconds, then on high speed for a full minute.

Once cookies are completely cooled, spread the frosting on each cookie and sprinkle with colorful sprinkles.

Store the frosted cookies at room temperature for up to one day or in the refrigerator for up to five days.