Step 1: Make the Tart Crust

1. In a large bowl, whisk together the flour, cocoa powder (if using), sugar, and salt.
2. Add the cold, cubed butter and use a pastry cutter or your fingers to work it into the dry ingredients until the mixture resembles coarse crumbs.
3. Mix in the egg yolk, then add ice-cold water one tablespoon at a time until the dough just comes together.
4. Shape the dough into a disc, wrap it in plastic wrap, and refrigerate for at least 30 minutes.
5. Roll out the chilled dough on a lightly floured surface to fit a 9-inch tart pan. Press it into the pan, trim any excess, and prick the bottom with a fork.
6. Blind bake the crust: Line it with parchment paper, fill with pie weights or dried beans, and bake at 350°F (175°C) for 15 minutes. Remove the weights and bake for another 5-7 minutes. Let it cool completely.

Step 2: Prepare the Chocolate Ganache

1. Heat the heavy cream in a small saucepan over medium heat until it just begins to simmer. Do not let it boil.
2. Pour the hot cream over the chopped chocolate and let it sit for 1-2 minutes.
3. Stir gently with a spatula or whisk until smooth and glossy. If adding butter and vanilla extract, mix them in until fully incorporated.

Step 3: Assemble the Tart

1. Pour the warm chocolate ganache into the cooled tart shell, spreading it evenly.
2. Let the ganache set at room temperature for about 10-15 minutes.

Step 4: Add the Cherry Topping

1. Arrange the cherries on top of the ganache, either randomly or in neat concentric circles for a polished look.
2. If desired, warm the cherry preserves slightly and brush them over the cherries for a glossy finish.

Step 5: Chill and Serve

1. Place the tart in the refrigerator for at least 2 hours to allow the ganache to firm up.
2. Before serving, let the tart sit at room temperature for 10 minutes for the best texture.
3. Slice and enjoy!