In a double boiler, combine chopped dark chocolate and heavy cream over simmering water. Stir gently until smooth, about 5 minutes. Remove from heat and pour into a shallow dish. Cool at room temperature, then refrigerate for at least 1 hour until set.

In a large mixing bowl, cream together softened unsalted butter, granulated sugar, and light brown sugar using an electric mixer on medium speed until fluffy and light in color, about 3-5 minutes. Mix in the egg and vanilla extract until well combined.

In a separate bowl, whisk together all-purpose flour, cocoa powder, baking soda, and salt. Gradually fold into the wet ingredients with a spatula until no visible flour streaks remain.

Cover the bowl with plastic wrap and refrigerate the dough for 1 hour.

Preheat your oven to 350°F (180°C). Scoop about 1.5 tablespoons of dough, flatten, place a piece of ganache in the center, and fold the dough over the ganache to seal.

Place cookies on a parchment-lined baking sheet, cover with plastic wrap, and refrigerate for an additional 30 minutes.

Ensure your oven is preheated to 350°F (180°C) while the cookies chill.

Remove cookies from the refrigerator and place them 2 inches apart on the baking sheet. Bake for 10-12 minutes until edges are set but centers look underbaked.

Let cookies cool on the baking sheet for 5 minutes before transferring to a wire rack. Dust with powdered sugar before serving.