For the mousse, place the chocolate in a medium heatproof bowl. In a medium saucepan, combine the heavy cream and coffee powder over medium high heat. When bubbles start to appear at the edges, but the mixture is not boiling, pour the hot cream over the chocolate and let it sit for 1 minute. Whisk until the chocolate is completely melted and stir in the salt. Cover and chill in the refrigerator for at least 8 hours.

For the caramel, place the granulated sugar in a medium saucepan and slowly pour over the water. Bring the mixture to a roiling boil over high heat without moving the pan. The sugar will start to caramelize after 3 to 4 minutes. When the edges are light brown, gently swirl the pan until the syrup turns deep golden brown. Keep a watchful eye over the pan because the sugar can burn very quickly.

Reduce the heat to low and slowly pour the heavy cream into the sugar syrup. Increase the heat to medium and whisk together until the caramel and cream are fully combined, about 2 to 3 minutes. Whisk in the butter, salt and vanilla extract.

Transfer the caramel to an airtight container and chill in the refrigerator for at least 4 hours up to 1 week.

For the tart shell, use a stand mixer fitted with the paddle attachment to cream the butter, sugar and salt until light and pale, about 2 to 3 minutes on medium speed. Use a spatula to scrape down the sides. On low speed, mix in the flour followed by the yolk until a dough forms. Press the dough into a disk and wrap tightly with plastic wrap. Chill in the refrigerator for at least 1 hour.

On a lightly floured work surface, roll out the dough until it is about 1/4-inch thick. Press the dough into a 10-inch tart pan with a removable bottom. Patch any holes or cracks as you go. Chill the shell in the refrigerator for 30 minutes.

Preheat the oven to 350 degrees F. Bake the tart shell for 30-35 minutes until it is light golden brown. Let cool completely to room temperature before filing the tart. Using a stand mixer fitted with the whip attachment, whisk the mousse until it holds soft peaks. Spread the mousse over the caramel. Drizzle the remaining caramel over the tart in a crisscross pattern. Sprinkle with milk chocolate shavings.

Before serving, chill the tart for at least 30 minutes. Store the tart in an airtight container for up to 8 hours.