Preheat oven to 350º F. Line cookie sheets with parchment paper or a nonstick baking mat.
Cream together butter and granulated sugar until light and fluffy, about 3 minutes. Add egg and mix until well-combined.
Stir in flour, cocoa powder, baking powder, salt, and vanilla.
Scoop cookie dough by the tablespoon full and roll into a ball.
Add granulated sugar to a large bowl for rolling cookie dough in before baking. Place each ball of cookie dough into the bowl of coarse sugar and roll to coat well. Place cookie dough onto baking sheet, spacing about 1½-inches to 2 inches apart. Lightly press each cookie down. Bake for 8 to 10 minutes or until lightly browned. Cool completely.
Chocolate Ganache Filling:
Prepare ganache filling according to recipe instructions and cool completely.
Spread about 1 teaspoon of ganache filling on the flat side of one cookie. Sandwich with another cookie.
Chocolate Ganache Cookies Recipe https://addapinch.com/chocolate-ganache-cookies-recipe/