In a mixing bowl, combine flour and sugar. Add cold butter and mix until crumbly.

Add egg yolk and ice water, mixing until the dough forms. Shape into a disk and refrigerate for 30 minutes.

Preheat the oven to 350°F (175°C). Roll out the dough and fit it into a tart pan.

Prick the bottom with a fork and bake for 20 minutes until golden. Let it cool.

Heat the heavy cream in a saucepan over medium heat until it just begins to simmer.

Remove from heat and add chopped chocolate, stirring until smooth.

Stir in softened butter until fully incorporated. Let it cool slightly.

Pour the ganache filling into the cooled tart crust, smoothing the top with a spatula.

Refrigerate for at least 2 hours until set.

Slice and serve chilled, optionally garnished with whipped cream or berries.