1-First Step: Prepare Wet Ingredients Beat ½ cup softened unsalted butter with 1 cup granulated sugar in a large bowl until light and fluffy, about 2-3 minutes. Add 1 large egg, ½ cup sour cream, and 1 tsp vanilla extract. Mix on medium speed until smooth. Scraping sides ensures even blending. Room-temperature ingredients prevent lumps.

2-Second Step: Combine Dry Ingredients Whisk 2 cups all-purpose flour, ½ cup unsweetened cocoa powder, 1 tsp baking soda, and ¼ tsp salt in a separate bowl. Gradually add dry mix to wet ingredients. Stir until a thick dough forms; do not overmix. Dough should be sticky but scoopable.

3-Third Step: Chill the Dough Cover bowl with plastic wrap and refrigerate for 1 hour. Chilling prevents spreading and enhances flavor. For quicker results, chill 30 minutes in freezer. This step is key for thick, chewy cookies.

4-Fourth Step: Scoop and Shape Line baking sheets with parchment paper. Scoop 2-tablespoon dough balls, roll smooth, and place 2 inches apart. Flatten slightly with palm for even rounds. Bake 10-12 minutes until edges set but centers look soft. Cookies firm as they cool.

5-Fifth Step: Cool Completely Transfer to wire racks. Let cool 20-30 minutes. Warm cookies crumble when frosted.

6-Sixth Step: Make Frosting Beat ½ cup softened butter, 2 cups powdered sugar, 1 tsp vanilla, and 2-3 tbsp milk until creamy. Adjust milk for spreadable texture. Spread 1-2 tbsp on each cooled cookie.

7-Final Step: Decorate and Serve Add sprinkles or chocolate chips immediately before frosting sets. Enjoy fresh for best texture. Store extras as noted below. Pairs great with cold milk. Total hands-on time: 30 minutes, plus chilling and baking.