Chocolate Peanut Butter Cookie Sandwiches

Preheat the oven to 350°F (175°C). Line baking sheets with parchment paper.
In a large bowl, cream the softened butter and granulated sugar on medium speed until light and fluffy, 2–3 minutes.
Beat in the egg and vanilla extract until smooth and glossy.
In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt to remove any cocoa lumps.
Add the dry ingredients to the wet on low speed just until a soft dough forms. If the dough seems sticky, let it rest 5 minutes or chill briefly.
Scoop tablespoon-sized portions of dough, roll into balls, and arrange 2 inches apart on the prepared sheets. Lightly flatten each ball with your palm or the bottom of a glass.
Bake 8–10 minutes, until the edges are set and centers look slightly soft. Cool on the sheet 5 minutes, then transfer to a wire rack to cool completely.
Make the filling: Beat the creamy peanut butter and softened butter until smooth. Gradually add powdered sugar and beat until fluffy. Add milk 1 teaspoon at a time until the filling is smooth and spreadable.
Pair cookies by size. Spread or pipe peanut butter filling onto the flat side of half the cookies. Top with the remaining cookies and press gently to form sandwiches.
Serve immediately or chill 15 minutes to help the filling set. Store in an airtight container (room temp up to 3 days, refrigerated up to 1 week, or freeze up to 2 months).