Line a 13x9-inch baking pan with foil, letting excess extend over sides of pan. Spray bottom of pan with baking spray with flour.
In the bowl of a stand mixer fitted with the paddle attachment, beat butter and brown sugar at medium speed until fluffy, 3 to 4 minutes, stopping to scrape sides of bowl. Add egg and vanilla, beating well.
In a medium bowl, whisk together flour, cocoa, baking powder, and salt. With mixer on low speed, gradually add flour mixture to butter mixture, beating just until combined and stopping to scrape sides of bowl. Press into bottom of prepared pan. Cover and refrigerate until firm, about 20 minutes.
Preheat oven to 350°F (180°C).
Bake until fragrant, bounces back when touched, and top looks dry, 10 to 15 minutes. Let cool completely in pan on a wire rack.
Spread Peppermint Cream Cheese Filling on top of cooled cookie base. Freeze until set, at least 2 hours, or until ready to use.
Pour warm Ganache on top of filling, and spread evenly. Garnish with candies, if desired. Freeze until set, about 1 hour.
Using foil as handles, remove from pan, and cut into bars. Let stand until slightly softened, about 30 minutes.
Recipe by Bake from Scratch at https://staging.bakefromscratch.com/chocolate-peppermint-dream-bars/