Preheat your oven to 350 degrees Fahrenheit. In a small bowl, combine flour and salt. In another bowl, cream together softened butter and sugar until the mixture is light and fluffy, about 2 minutes with an electric mixer. Add the egg yolk and mix until fully incorporated. Gently fold in the flour mixture until just combined, being careful not to overwork the dough.

Divide the dough among 8 mini tart pans with removable bottoms, pressing it evenly into the bottom and up the sides. Use the bottom of a measuring cup to create a flat, even surface. Place the filled tart pans on a large baking sheet and bake for 12 to 15 minutes until the shells are golden brown and set but not darkened. Remove from the oven and let cool for 5 minutes before carefully removing from the pans. The shells should still be warm, making removal easier.

While the tart shells cool, place chopped dark chocolate in a heatproof bowl. In a small saucepan, heat the heavy cream over medium heat until it just begins to simmer (small bubbles forming around the edges). Pour the hot cream over the chocolate and add the butter. Let it sit undisturbed for 2 minutes, then whisk until smooth and glossy. Allow the ganache to cool for 3 to 4 minutes so it reaches a pourable consistency but is still warm.

Carefully pour or spoon the warm ganache into each cooled tart shell, filling to just below the rim. Smooth the surface with the back of a small spoon if desired. Let the tarts sit at room temperature for 10 minutes to allow the ganache to set slightly, then refrigerate for at least 30 minutes until the chocolate is completely set and firm to the touch.

Just before serving, top each tart with one or two fresh berries and a small mint leaf if using. Arrange the finished tarts on a decorative serving platter and serve chilled or at room temperature. For make-ahead convenience, prepare the tarts up to 8 hours in advance and add fresh berries just before serving to maintain their appearance and texture.