For Cookies:

3/4 cup all-purpose flour, plus more for dusting

1/3 cup unsweetened Dutch-process cocoa powder

1/8 tsp salt

6 Tbsp (3/4 stick) unsalted butter, softened

3/4 cup confectioners' sugar, sifted

1 large egg

1/2 tsp vanilla extract

 

Line baking sheets with parchment paper; lightly coat with cooking spray.  Whisk together flour, cocoa, and salt.  With an electric mixer on medium-high speed, cream butter and confectioners' sugar until pale and fluffy.  Beat in egg and vanilla.  With mixer on low speed, gradually add flour mixture, beat until combine.  Divide dough in half; form into disks, and wrap in plastic.  The dough will be very sticky.  Chill until firm, at least 2 hours, or up to 2 days.  Dough can also be frozen up to 2 months; thaw in refrigerator before using.

 

Working with one disk at a time; roll out dough to a 1/8-inch thickness on a lightly floured surface.  Using a 1-inch round biscuit or cookie cutter, cut out shapes, transferring to prepared sheets (1 inch apart) as you work.  Chill in freezer until firm, about 15 minutes.  Brush off excess flour.  Gather togther scraps; roll, chill, and cut out more shapes.

 

Preheat oven to 325.  Bake cookies, rotating sheets halfway through, until edges are firm, about 12 to 14 minutes.  Transfer cookies, on parchment, to a wire rack to cool completely.

 

Spread a pipe a small amount of filling on half the cookies.  Top with remaining cookies; gently press down to spread filling to edges.  Assembled cookies can be refrigerated up to 2 days in an airtight container.

 

For Vanilla Filling:

6 Tbsp (3/4 stick) unsalted butter, softened

1 1/4 cup confectioners' sugar

1 tsp vanilla extract

 

With an electric mixer on medium-high speed, beat butter until smooth.  Add confectioners' sugar, and beat until light and fluffy.  Use immediately.

 

For Peanut Butter Filling:

Mix as above, but omit the vanilla extract.  Beat in 1/4 cup smooth peanut butter and 1 Tbsp milk into filling.