Rich, velvety chocolate ganache in a chocolate cookie crust.
Chocolate Curl Garnish
Special equipment: an 11-inch fluted tart pan with a removable bottom
Add the cookies to a food processor and pulse to fine crumbs. Pour in the melted butter and pulse together. Tip into an 11-inch fluted tart pan with a removable bottom and press the mixture in with a 1/4-cup measure to form an even crust on the bottom and sides. Put it in the freezer for 15 minutes to set.
Put the cream and butter in a pan and heat until the mixture starts to bubble.
Put the dark and milk chocolate in a bowl and pour the warmed cream and butter over it. Mix everything together until melted and fully combined, then pour it into the chilled tart base. Refrigerate the tart overnight until set.
Put the white chocolate and coconut oil in a bowl and set over a pan of simmering water until the chocolate is melted; mix together. Pour the melted mixture onto the underside of a clean baking sheet and spread it in a very thin layer with an offset spatula or knife. Put it in the freezer until set, about 2 minutes. (To check if it is set, press it with your fingertip; it should leave the slightest mark but not a depression.)
Using a sharp-edged spatula, scrape the chocolate from the bottom of the baking sheet into curls. Remove the tart from the pan and decorate with the curls.