For the cookie crust: Preheat oven to 350 degrees F. Line a baking sheet with a Silpat or with foil. Set aside an 8 inch fluted tart pan with a removable bottom.
Combine butter and sugar in bowl of stand mixer. Cream for about 3 minutes until light and fluffy.
Combine the flour, cocoa powder, and salt in a medium bowl.
Add to stand mixer and beat until combined.
Mixture will be loose and crumbly.
Pour out mixture on prepared baking sheet and spread evenly.
Bake for about 20 minutes, rotating halfway. The cookie crumbles will start smelling very fragrant.
Remove from oven and let cool on wire rack.
Place cookies into food processor and pulverize into even, fine crumbs.
Pour out crumbs into a medium bowl and add the melted butter. Stir with a spoon to combine.Press crumb mixture evenly into bottom and up sides of tart pan.
Bake tart shell for about 10 minutes in the 350 degree F oven.
Place white chocolate in a medium bowl. Bring cream to a boil on the stove and pour over the white chocolate.
Let sit for a minute and stir to fully combine chocolate and cream. If the chocolate does not fully melt, you can place the bowl over a saucepan of simmering water and stir until combined. Do not leave on too long or the chocolate could burn.
Add in the peppermint extract and butter and stir until combined.
Place dark chocolate in a medium bowl.Bring cream to a boil on the stove and pour over the chocolate.
Let sit for a minute and stir to fully combine chocolate and cream. If the chocolate does not fully melt, you can place the bowl over a saucepan of simmering water and stir until combined. Do not leave on too long or the chocolate could burn.
Add in the butter and stir until combined.Carefully pour mixture into tart shell over the white chocolate filling. Refrigerate for about an hour until set.