Crush the chocolate cookies into fine crumbs. A food processor is fastest, but a ziplock bag and rolling pin work well too.

Melt the butter and mix it thoroughly with the cookie crumbs until the mixture resembles wet sand.

Press the buttery crumbs firmly into a 9-inch tart pan with a removable bottom. Use the back of a spoon or a measuring cup to compact them evenly across the bottom and up the sides.

Chill the crust in the refrigerator for at least 30 minutes. This helps it set so it will not crumble when slicing.

While the crust chills, make the filling. Finely chop the dark chocolate and place it in a heatproof bowl.

Heat the heavy cream in a saucepan over medium heat until it just begins to simmer. Watch carefully to prevent boiling, which can split the cream.

Pour the hot cream over the chopped chocolate and let it sit undisturbed for 2 to 3 minutes to allow the chocolate to melt.

Stir gently with a rubber spatula, starting from the center and working outward, until the mixture becomes smooth and glossy.

Sift in the cocoa powder and powdered sugar, then add the vanilla extract. Whisk until fully incorporated and no lumps remain.

Remove the chilled crust from the refrigerator and pour the chocolate filling into it. Gently tap the pan on the counter to remove any air bubbles.

Refrigerate the tart for at least 4 hours, preferably overnight, until completely set. The filling should be firm to the touch.

Just before serving, decorate the cooled tart with chocolate shavings and a sprinkling of cacao nibs for texture.

To serve, run a sharp knife under hot water, dry it, and make clean cuts. Wipe the knife between slices for neat presentation.