In a large mixing bowl, whisk together flour, sugar and salt. Add cold butter cubes. Toss with flour mixture. Using your finger tips or a pastry cutter, break the butter into small pieces, no larger than the size of pea. The mixture should look like coarse breadcrumbs. Transfer the bowl in the fridge to chill.
In the meantime, in a small bowl, whisk together egg yolk, vanilla extract and milk.
Add the wet mixture into the flour mixture. The mixture will be crumbly. Using your hand, squeeze the dough together and knead gently for a couple times. Shape into a round disk. Wrap in plastic wrap and chill in the fridge for 15 minutes.
Lightly grease the bottom and sides of a 9-inch round tart pan (with a removable bottom) with butter.
In a lightly floured surface, roll the dough into about 1/8-inch thick. While working, dust with more flour to prevent sticking. Carefully roll the dough onto a rolling pin and transfer to the prepared tart pan. Press and fit the dough into the pan. Trim off any excess dough. If there is any crack or hole, patch with a thin layer of extra dough. Prick the dough all over with a fork (just the bottom, not the sides). Chill the dough in the fridge for at least 30 minutes.
Preheat the oven to 375˚F (190˚C). Prepare a 12 x 12 parchment paper, crunch it up roughly and open it back up, so it can be easily shaped into the tart.
Place and fit the parchment paper on the tart. Fill with pie weights or dry beans. Bake in the oven for 12 – 14 minutes, until the crust feels dry and set. Carefully remove the pie weights and parchment paper. Bake in the oven for 8 – 10 more minutes, until golden brown.
Remove the crust from oven and cool completely on a cooling rack (keep the tart in the pan).
For the ganache, in a large mixing bowl, place chopped chocolate.
In a small saucepan over medium heat, combine heavy cream and sugar. Cook until it just starts to simmer. Pour over the chocolate immediately. Set aside for 5 minutes. Add orange zest and softened butter cubes. Stir with a spatula until smooth and glossy. Pour the ganache into the cooled tart crust (still in tart pan). Remove the air bubbles with a toothpick. Carefully transfer the tart to the fridge to set for at least 1 hour.
When ready to serve, carefully remove tart from pan. Sprinkle the top with flaky sea salt. Garnish with chocolate shaving and orange slices if using. Cut the tart with a hot knife. Serve cold.