Preheat oven to 375°F (190°C). In a food processor, pulse together flour, cocoa powder, powdered sugar, and salt.

Add the cold cubed butter and pulse until the mixture resembles coarse crumbs.

Add ice water, 1 tablespoon at a time, pulsing until the dough just begins to clump together.

Press the dough evenly into the bottom and up the sides of a 9-inch tart pan. Prick the bottom with a fork. Chill for 30 minutes.

Bake for 20-25 minutes, or until the crust is firm and fragrant. Let it cool completely.

In a medium saucepan, combine the raspberries, sugar, and lemon juice. Cook over medium heat, stirring occasionally, until the raspberries break down (about 5-7 minutes).

In a small bowl, whisk the cornstarch with 2 tablespoons of cold water to create a slurry. Pour it into the raspberry mixture and whisk to combine.

Bring the mixture to a simmer and cook for 1-2 minutes, stirring constantly, until it has thickened. Remove from heat and let it cool for 15 minutes.

Spread the cooled raspberry filling evenly over the bottom of the cooled tart crust.

To make the ganache, place the finely chopped chocolate in a heatproof bowl. Heat the heavy cream in a saucepan over medium heat until it just begins to simmer. Pour the hot cream over the chocolate and let it sit undisturbed for 5 minutes.

Whisk the mixture gently until the ganache is smooth, shiny, and completely combined.

Pour the ganache over the raspberry layer, smoothing the top with a spatula.

Refrigerate the tart for at least 4 hours, or until the ganache is firm. Garnish with fresh raspberries before serving.