Step 1: In a food processor, pulse flour, cocoa powder, and sugar. Add cold butter and pulse until the mixture looks like coarse crumbs.
Step 2: Add the egg yolk and ice water, pulsing until the dough just begins to come together.
Step 3: Turn the dough onto a floured surface, form into a disc, wrap in plastic, and chill for at least 1 hour.
Step 4: Roll out the dough and fit it into a 9-inch tart pan. Prick the bottom with a fork and freeze for 15 minutes.
Step 5: Line the tart with parchment paper and pie weights. Bake at 375°F (190°C) for 15 minutes, then remove weights and bake for another 5 minutes. Let cool completely.
Step 6: Place chopped chocolate in a heat-proof bowl. Heat heavy cream in a small saucepan until it reaches a simmer.
Step 7: Pour the hot cream over the chocolate and let sit for 5 minutes. Add salt and vanilla, then whisk gently until smooth.
Step 8: Pour the chocolate ganache into the cooled tart shell. Smooth the top and refrigerate for at least 4 hours, or until set.
Step 9: Before serving, decorate the howl chocolate tart recipe with a wolf stencil or sea salt flakes.