In a large mixing bowl, add the softened unsalted butter and vegetable oil. Beat them together with granulated sugar and brown sugar until light and fluffy.

Incorporate the egg and vanilla extract, stirring until well mixed.

In another bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking soda, and salt.

Gradually add the dry mixture to the wet dough, blending gently until well combined.

Cover the bowl with plastic wrap and refrigerate for 2 hours.

Preheat your oven to 350°F (177°C).

Once the dough is chilled, scoop it out and roll it into balls, placing them on a lined baking sheet.

Bake for 12-13 minutes.

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

For the frosting, beat the softened butter until creamy.

Gradually add confectioners' sugar, cocoa powder, heavy cream or milk, vanilla extract, and a pinch of salt, mixing until smooth.

Once the cookies have cooled, frost them generously with the chocolate frosting and top with colorful sprinkles.