Preheat the oven to 350°F (175°C).
In a mixing bowl, whisk together the flour, cocoa powder, baking soda, and salt.
In another bowl, mix melted butter, brown sugar, and granulated sugar until smooth.
Add the egg and vanilla extract to the sugar mixture, stirring until well combined.
Gradually, add the dry ingredients to the wet mixture, stirring gently until just combined.
Fold in the chocolate chips and sprinkles.
Scoop tablespoon-sized portions onto a lined baking sheet, spacing them about 2 inches apart.
Bake for 10-12 minutes or until the edges are set but the centers remain soft.
Let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack.
Once completely cooled, store in an airtight container.