Preheat your oven to 350°F (175°C).

In a mixing bowl, combine the all-purpose flour, cocoa powder, sugar, and salt. Mix well to avoid any lumps.

Add the cold, cubed butter to the dry mixture. Mix until the mixture resembles coarse crumbs.

Add the egg yolk and cold water, one tablespoon at a time, mixing until the dough comes together.

Roll the dough out slightly and press it into muffin tins or tart pans, ensuring it reaches the edges and corners. Prick the bottoms with a fork to prevent puffing.

Bake for 15 minutes, then let cool completely.

In a small saucepan over low heat, combine the chocolate chips and heavy cream. Stir continuously until the chocolate is melted and the mixture is smooth.

Remove from heat and add vanilla extract, mixing thoroughly. Let the filling cool slightly before pouring into the cooled crusts.

Once the filling is slightly cooled, carefully pour it into each tart shell.

Chill the tarts in the refrigerator for at least 30 minutes to set the fudge.

Garnish with your choice of toppings and serve.