Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper or a silicone baking mat.
Cream together the softened unsalted butter, granulated sugar, and packed dark brown sugar until light and fluffy, about 2-3 minutes with an electric mixer.
Beat in the large egg and vanilla extract until fully combined and glossy.
In a separate bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking soda, and salt until no lumps remain.
Gradually add the dry ingredients to the wet ingredients, mixing just until combined. Avoid overmixing.
Gently fold in the freshly grated grapefruit zest, cold-fashioned oats, Cheerios, and semi-sweet chocolate chips.
Drop tablespoon-sized dollops of dough onto the prepared baking sheet, spacing about 2 inches apart. Bake for 10-12 minutes until edges are set but centers are slightly soft.
Allow the cookies to cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.