Preheat the oven to 375 degrees F.
Place tart shells on a parchment paper-lined baking sheet. Poke the bottom of each tart with a fork a few times to prevent them from puffing as they bake. Bake 10 minutes. Remove tart shells from the oven and cool a few minutes. Leave oven at 375 degrees F.
Place the eggs in a medium bowl and beat until the whites and yolks are well blended. Mix in the lemon juice, sugar and butter. Divide and pour this mixture into the tart shells, filling them as close to the top as possible. Bake the tarts for 15 minutes, or until the filling is set. Cool tarts to room temperature and then refrigerate for at least an hour. Carefully remove the tin-foil liners and place tarts on a platter. Cover and refrigerate tarts until ready to serve.
When ready to serve, lightly dust with icing sugar, garnish with mint sprigs, if desired, and enjoy.
Note: Mini tarts shells come with 18 in a box, so you’ll need two boxes to get the required amount for this recipe. Keep the remaining tart shells frozen for another use.