Begin by melting the dark chocolate chips in a microwave-safe bowl. Heat in 20-second intervals, stirring in between until fully melted and smooth. Mix in the heavy cream until well combined, creating a luscious chocolate filling. Chill for about 30 minutes, or until firm enough to scoop.

In a large mixing bowl, cream together the softened unsalted butter and granulated sugar using an electric mixer until fluffy and pale, about 2-3 minutes. Add in the vanilla extract and salt, mixing well. Gradually incorporate all-purpose flour and cocoa powder, mixing on low speed until a cohesive dough forms.

Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper. Scoop out about a tablespoon of cookie dough, flatten it slightly, and place a teaspoon of the chilled chocolate filling in the center. Fold the edges over the filling and roll it back into a ball.

Roll each cookie ball in the finely crushed mixed nuts until completely covered, pressing the nuts in slightly to stick during baking.

Arrange the nut-coated cookie balls on the baking sheet, leaving space between them. Bake for 10 to 12 minutes until set but slightly soft in the center.

Once baked, let the cookies cool on the baking sheet for about 5 minutes, then transfer to a wire rack to cool completely. Dust with powdered sugar before serving if desired.