Melt the chocolate and butter together until smooth, either in a microwave (20–30 second bursts) or using a double boiler. Stir and set aside to cool slightly.

In a separate bowl, whisk the eggs and sugar for 2–3 minutes, until the mixture becomes light and fluffy. Slowly pour in the cooled chocolate mixture and vanilla extract (if using), and stir to combine.

Add the flour, baking powder, and salt. Mix just until you get a thick, glossy batter. Cover and refrigerate the dough for 30 to 60 minutes to firm up.

Preheat the oven to 350°F (180°C) and line a baking sheet with parchment paper. Scoop the chilled dough into small balls, about the size of a walnut, and place them on the tray with space in between.

Bake for 10 to 12 minutes. The cookies should be set around the edges but still soft in the center. Let them cool on the tray for 10 minutes, then transfer to a wire rack to cool completely.

While the cookies are cooling, melt the chocolate for the mousse and let it cool slightly. In a cold bowl, whip the heavy cream with powdered sugar until stiff peaks form.

Gently fold the melted chocolate into the whipped cream in batches. Stir carefully until fully combined and smooth. Refrigerate the mousse for 15 to 30 minutes to thicken slightly.

Match the cookies by size. Spoon or pipe a small amount of mousse onto the bottom of one cookie, then press another cookie on top to make a sandwich.

Place the filled cookies in the fridge for 1 to 2 hours, until the mousse is fully set and the cookies are ready to serve.