1. To make the tart dough: Put the flour, powdered sugar, and salt in a food processor and pulse until combined. Drop the butter pieces over the dry ingredients and pulse until the butter is coarsely cut in. Break up the egg yolk with a fork and add a little at a time, pulsing after each addition. When the entire egg yolk is in, process in long pulses, about 10 seconds each, until the dough forms clumps. Turn the dough out onto a work surface and gently knead to form a ball.

2. Butter a 9-inch fluted tart pan with a removable bottom. Press the dough evenly over the bottom and up the sides of the pan. Don't press too hard, you don't want the crust to lose its crumbly texture. Freeze the crust for at least 30 minutes, or longer, before baking.

3. To make the key lime filling: Preheat the oven to 350 degrees F. In a medium bowl, combine sweetened condensed milk, lime zest, key lime juice, and egg yolks. Whisk until smooth. Remove tart dough from the freezer. Pour mixture into tart pan. Bake until filling is set around edge but still slightly loose in center and crust is golden brown, about 30 minutes.

4. Cool tart completely at room temperature; then refrigerate until chilled, at least 2 hours before serving.