In a mixing bowl, whisk together the all-purpose flour, sugar, and kosher salt. Cut in the cold unsalted butter until you achieve a crumbly texture.

In a separate bowl, combine the egg yolk, vanilla extract, and whole milk. Gradually mix these wet ingredients into the dry mixture until it starts to come together.

Chill the dough for 15 minutes, then roll it out and fit it into a greased 9-inch tart pan. Chill for another 30 minutes, and prick the base with a fork.

Preheat your oven to 375°F (190°C). Line the tart crust with parchment paper and add pie weights. Bake for 12-14 minutes until edges start to set.

Remove the weights and parchment, continue to bake for an additional 8-10 minutes until golden brown. Allow to cool completely.

Chop the semi-sweet chocolate and place it in a medium bowl. Heat the heavy cream and sugar in a saucepan until it simmers.

Remove from heat and pour over the chopped chocolate. Let it sit for 5 minutes, then stir in the orange zest and unsalted butter until smooth.

Pour the chocolate-orange ganache into the cooled tart shell and spread evenly with a spatula. Chill in the refrigerator for at least 1 hour.

Before serving, enhance with a sprinkle of flaky sea salt, chocolate shavings, and fresh orange slices. Serve chilled to impress your guests.