Make the cookies:

1. In a medium mixing bowl, whisk together flour, cocoa powder, baking powder, and salt. Set aside.
2. Use a stand mixer (or hand mixer) and cream the butter and sugars together in a mixing bowl on medium speed until light and fluffy, about 2-3 minutes.
3. Add egg and vanilla and continue to beat on medium until fully incorporated.
4. Gradually add the dry ingredients until just combined.
5. Place granulated sugar for rolling into a shallow bowl. Take 1 tablespoon of dough at a time and roll into 1-inch balls. Roll into the sugar to coat.
6. Make an indentation with your thumb (or the back of a small spoon or melon baller) into the centre of each ball.
7. Place cookies on a silpat or parchment-lined half sheet baking pan, spaced 2 inches apart. Transfer the baking sheet to the refrigerator and chill the cookies for 1 hour (or place in the freezer for 30 minutes).
8. Preheat the oven to 350 F.
9. Bake the cookies for 10-13 minutes until the edges are set. If the indentations have started to close, use the back of a spoon or melon baller to carefully make the indentation bigger again, while the cookies are still hot.
10. Allow the cookies to cool on the baking sheet for 5 minutes, then carefully transfer to a wire cooling rack to cool completely.

Make the ganache filling:

1. Place chopped chocolate into a medium mixing bowl.
2. Heat the heavy cream in a small saucepan until it begins to simmer. Immediately remove from heat before it begins to boil.
3. Pour hot cream over top of the chopped chocolate and let it sit for 5 minutes to allow it to soften the chocolate. Then use a spoon to slowly stir until completely combined. Allow the ganache to cool down and thicken uncovered for 10-20 minutes.
4. Use a small spoon to scoop just under a teaspoon of the ganache into each indentation.
5. Sprinkle crushed pistachios on top of the ganache.
6. Allow the ganache to set completely (about 1 hour or faster in the refrigerator) before storing the cookie to avoid smudging.