Preheat oven to 350°F (175°C). Combine flour and sugar in a bowl.
Cut in cold diced butter using a pastry cutter or fingertips until mixture resembles coarse crumbs.
Press mixture evenly into the bottom and sides of a 9-inch tart pan and prick the base with a fork.
Bake crust for 18 to 22 minutes until lightly golden. Let cool completely.
Melt butter in a saucepan over medium heat. Add brown sugar and cook for 2 to 3 minutes until dissolved and bubbling.
Slowly whisk in heavy cream and simmer for 1 to 2 minutes until slightly thickened.
Remove from heat. Stir in sea salt flakes and vanilla extract.
Whisk egg yolks in a separate bowl. Temper yolks by slowly whisking in a few spoonfuls of warm caramel.
Pour yolk mixture back into saucepan and cook gently for 2 to 3 minutes, stirring until thickened but not boiling.
Pour caramel filling into cooled tart crust and smooth the surface. Chill for 30 to 45 minutes until set.
Place chopped chocolate in a heatproof bowl. Heat heavy cream until just simmering.
Pour hot cream over chocolate and let sit for 1 to 2 minutes, then stir until smooth and glossy.
Pour ganache over chilled caramel layer and smooth the top.
Refrigerate tart for at least 1 to 2 hours until ganache is fully set.
Before serving, garnish with flaky sea salt, chocolate shavings, and a light dusting of cocoa powder.