Line 2 baking sheets with parchment paper. Set aside.
In a large bowl, whisk together flour, cocoa powder, baking powder and salt. Set aside.
Beat butter and both sugars with an electric mixer until light and creamy. Scrape down the sides of the bowl with a spatula as needed.
Add egg and vanilla then beat until incoporated.
Reduce mixer speed to low, add dry ingredients to wet and mix until just combined.
Using a 1-tablespoon sized cookie scoop, scoop dough into round balls then place on a parchment-lined cookie sheets spaced 2 inches apart.
Using with the backside of a 1/4 teaspoon measuring spoon (or your thumb), make an indent in the top of each cookie.
Place baking pans in the refrigerator and allow dough to chill for a minimum of 2 hours.
Preheat oven to 350 degrees.
Bake cookies for 10 to 12 minutes until the edges are set. Avoid overbaking or cookies will be dry.
Remove baking sheet from the oven then deepen the indents with the backside of a 1/4 teaspoon measuring spoon (or your thumb).
Allow cookies to cool on the baking sheet for 5 minutes then transfer cookies to a wire rack to cool completely before filling with ganache.