In the bowl of a stand mixer fitted with the paddle attachment, or in a large mixing bowl using a handheld mixer, beat the butter until smooth. Add the brown sugar and continue mixing for 1 to 2 minutes or until well combined.

Mix in the egg and vanilla extract until fully combined.

In a separate mixing bowl, sift together the flour and cocoa powder, then whisk in the baking powder and salt.

Add the dry ingredients to the wet ingredients and mix until fully combined. The mixture will be a little crumbly at first, but it will come together as you continue mixing it. If needed, use a rubber spatula or your hands to finish working the cookie dough together.

Using a one-tablespoon cookie scoop or measuring tablespoon, measure out the cookie dough. Roll each one into a smooth ball, then roll in the granulated sugar (optional), and place onto the prepared baking sheets. Make sure to leave a little room between each one because they will spread some as they bake in the oven.

Using the bottom of a ½ teaspoon measuring spoon, gently press an indentation into the center of each ball of cookie dough. If the cookie dough cracks a little around the edges, you can smooth it back out with your fingers.

Place the baking sheets with the cookie dough in the fridge to chill for at least 30 minutes.

Preheat the oven to 375°F (190°C).

Remove the baking sheets from the fridge and bake for 8 to 10 minutes or until the cookies are set on top (I usually bake mine for 9 minutes).

Remove from the oven and use the bottom of a teaspoon measuring spoon to press an indentation back into the cookies. Allow to cool for 5 to 10 minutes on the baking sheets, then transfer to a wire rack to cool completely.

Pour the heavy whipping cream into a microwave safe bowl (or a glass measuring cup) and heat in the microwave for 25 to 30 seconds or until warmed through. Make sure to keep an eye on it so it doesn't bubble over in the microwave.

Pour the warm heavy whipping cream over the chocolate, cover with foil, and let it sit for 3 minutes.

Start whisking slowly in the middle of the bowl and continue whisking in one direction until the mixture is smooth and well combined. If the chocolate is not completely melted, microwave in 10 to 15 second increments until melted and smooth.

Spoon about 1 teaspoon of chocolate ganache into the center of each cookie. Allow the ganache to set up at room temperature or refrigerate for 15 minutes.