Place flour and butter in a food processor and pulse, until mixture resembles fine great crumbs. Add egg yolk and water and pulse until it forms a ball of dough. Transfer to a surface lined with plastic wrap, form a disc and refrigerate for 30 minutes.

Remove from the fridge and place on a lightly floured surface. Roll dough, until it is about 5 mm thick. Cut dough using the top of the tartlet tins (the amount of dough cut from 1 tin is enough to fill it and make 1 relatively thin tartlet shell. Spray tins with cooking spray very lightly.

Line eight 4 inch (9.5 cm) with dough,press into the sides. Refrigerate for 15 minutes, until firm.

Place tartlet tins on a baking sheet.Add baking weights, if you are using any. Cover with parchment paper. Bake at 350 F (170 C) for 15 minutes, then remove parchment paper and then bake for 10 more minutes. Carefully remove weights and bake for 3 more minutes, until golden.

Let tartlet shells cool completely and then remove form the tins.

In a medium bowl using a rubber spatula mix together Mascarpone cheese and ¼ cup sugar. (Do not mix Mascarpone with an electric mixer, because depending on its temperature and the room temperature it may separate)!

In the bowl of an electric mixer on high speed beat heavy cream until stiff peaks form. (Do not over beat, as it may turn into butter).

Using a rubber spatula gently fold whipped cream into Mascarpone-sugar mixture. Add lemon zest and mix to combine.

Combine blueberries and sugar in a medium sauce pan and cook over medium heat for 10-12 minutes. Add cornstarch, lemon juice, lemon zest and vanilla. Thin sauce with water if it appears too-thick. Store in an air tight container in the fridge until ready to use (or up to 5 days).

To assemble the tartlets:

Fill tartlet shells with mascarpone whipped cream and top with 1 tbsp blueberry sauce. Decorate with mint leaves. Serve immediately, or refrigerate for up to 4 days.