Crush approximately 24 Biscoff cookies into fine crumbs using a food processor or a plastic bag and a rolling pin. Mix with 7 tablespoons melted unsalted butter and press into a 9-inch tart pan. Chill for 10-15 minutes.

Heat 1 cup of heavy cream in a small saucepan until steaming. Remove from heat and pour over 8 ounces of chopped dark chocolate in a heatproof bowl. Let sit for 2-3 minutes, then stir until smooth.

Pour the ganache into the chilled tart crust and spread evenly. Drizzle with Biscoff cookie butter if desired, and chill for at least 4 hours until set.

Remove the tart from the pan and slice with a warm knife. Serve chilled, optionally garnished with whipped cream or crushed Biscoff cookies.