In a food processor, pulse flour, powdered sugar, and salt. Add cold butter and pulse until mixture resembles coarse crumbs. Add egg yolk and pulse, then add ice water 1 tablespoon at a time just until the dough comes together.
Divide dough into 12 equal portions.
Press into the bottom and sides of mini tartlet pans (or a mini muffin tin).
Prick bottoms with a fork. Chill for 30 minutes.
Preheat oven to 375°F.
Bake tart shells for 10–12 minutes or until lightly golden. Let cool completely.
Heat cream in a small saucepan until just simmering.
Remove from heat and add chocolate and butter.
Let sit 1 minute, then stir until smooth.
Stir in vanilla.
Spoon ganache into cooled tart shells.
Chill for 1 hour or until set.
Serve as is or top with berries, sea salt, chopped nuts, or whipped cream.