Mix the softened butter, granulated sugar, and brown sugar together with a spoon, fork, or small silicone spatula until creamed. (Or use a handheld or stand mixer fitted with a paddle attachment on medium speed—here’s a helpful tutorial if you need guidance on how to cream butter and sugar.) Stir in the beaten egg and vanilla.

In a separate bowl, whisk the flour, cocoa powder, baking soda, and salt together. Pour the dry ingredients into the wet ingredients and mix to combine. Fold in 1/4 cup chocolate chips.

Preheat oven to 350°F (177°C). Place dough in the refrigerator as oven preheats, even if it’s for only a few minutes.

Line a baking sheet with parchment paper or silicone baking mat. Spoon the dough in the center of the baking sheet and mold into a tall ball or mound using a spoon (see picture above). Dot the top of the cookie with 1 Tablespoon of chocolate chips. Add sprinkles on top, if desired.

Bake for 18–22 minutes, or until the edges appear set. The cookie will look super soft in the center, but will firm up as it cools.

Cool the cookie on the baking sheet for 5–10 minutes before eating. Store leftovers (if there are any!) covered at room temperature for up to 5 days. Cookie freezes well up to 3 months. Thaw at room temperature.