How to Make It
Prepare the Chocolate Sugar Cookie Dough: Beat butter and sugar with a heavy-duty electric stand mixer on medium-high speed until light and fluffy, about 3 minutes. Add eggs and vanilla. Beat on medium until combined, stopping to scrape down sides of bowl as needed.
Stir together flour, cocoa, baking powder, and salt in a medium bowl. With mixer running on low speed, add flour mixture to butter mixture, beating until just incorporated.
Shape dough into a disk, wrap tightly in plastic wrap, and chill at least 2 hours or up to 2 days.
Prepare the Bar Cookies: Preheat oven to 350°F. Lightly grease an 8-inch square baking pan with cooking spray. Line bottom and sides of pan with parchment paper, allowing 2 to 3 inches to extend over 2 sides. 5. Press chilled dough into an even layer in prepared pan. Bake in preheated oven until dry to the touch, about 22 minutes. Cool completely in pan on a wire rack, about 1 hour. Lift baked cookie dough from pan using parchment paper as handles, and place on a cutting board.
Prepare the Frosting: Beat unsalted butter and cream cheese with a heavy-duty electric stand mixer on medium speed until creamy, 1 to 2 minutes. Reduce speed to low, and, with mixer running, gradually add powdered sugar, beating until smooth. Add heavy cream and vanilla extract, and beat on medium-high speed until blended, about 20 seconds.
Spread frosting on baked cookie dough and cut into 18 triangles. Sprinkle with nonpareils.