In a mixing bowl, cream together caster sugar and cubed butter until light and fluffy, about 2-3 minutes. Crack in the egg and whisk until fully combined. Slowly add the plain flour, mixing gently with a spatula until a dough forms.

Lightly knead the dough on a floured surface just until smooth, then wrap it tightly in cling film. Refrigerate for at least 30 minutes to firm up the dough.

After chilling, unwrap the dough and roll it out to about 5mm thick. Use a round cutter to cut discs that fit your tart tins.

Remove the tart tins from the fridge and place them in a preheated oven. Bake for 12 minutes, or until the crust is golden brown.

In a heatproof bowl set over simmering water, combine double cream and butter. Stir until melted and then gradually stir in finely chopped chocolate until smooth.

Carefully pour the ganache into each cooled tart shell, ensuring an even fill. Let chill in the fridge for about 20 minutes.

Once set, gently remove the tarts from their tins and slice into pieces. Serve chilled, garnished with fresh berries or whipped cream.