Preheat the oven to 350°F (175°C). Line baking sheets with parchment.

In a large bowl, whisk the cake mix, 2 Tbsp. cocoa powder, 2 eggs, and 1/3 cup oil until smooth. The batter will be thick.

Drop rounded tablespoons (or use a cookie scoop) onto the prepared sheets, spacing about 2 inches apart.

Bake for 8–10 minutes, until edges are set but centers are soft. Do not overbake.

Let cookies cool on the sheet for 2 minutes, then transfer to a wire rack to cool completely before frosting.

In a medium bowl, beat 1/2 cup room-temperature unsalted butter until creamy.

Add 1 Tbsp. cocoa powder and 3 Tbsp. milk; beat to combine.

Gradually add 2 cups confectioners’ sugar, beating on low until incorporated, then increase speed until smooth and spreadable. Add a teaspoon more milk if too thick.

Spread or pipe a generous layer onto each cooled cookie. Allow frosting to set for 15–20 minutes before stacking.