Use melted butter instead of oil when mixing the cake mix base to achieve fudgier, softer cookies with better texture and prevent over-drying during baking.

Start with ½ teaspoon of peppermint extract and taste as you go, since it's potent—too much will overpower the chocolate and create a medicinal flavor.

Ensure your softened butter is between 65-70°F before creaming with powdered sugar; this prevents a greasy filling and creates a smooth, spreadable mint buttercream that holds its shape.

Let cookies cool on the baking sheet for 5 minutes before transferring to a wire rack; this sets their structure just enough to prevent cracking while maintaining the fudgy center.

Add food coloring to the buttercream last and mix thoroughly, as liquid coloring can slightly thin the filling—compensate by using slightly less milk in the initial creaming stage.