In a small saucepan, bring milk to a simmer. In a bowl, whisk sugar, eggs, yolks, & cornstarch together until smooth. When milk is simmering, whisk half of it into the egg mixture then gradually add the egg/milk mixture to the rest of the milk.
Return saucepan to the heat & cook, whisking constantly until very thick. Whisk in the butter & vanilla. Cover with plastic wrap & refrigerate. Be sure the plastic is touching the top of the custard to prevent it from forming a film over it. When custard is cooled & you are ready to use it, whip with an electric mixer for a couple of minutes.
Shortbread Crust
Preheat oven to 350 F. In a large bowl, combine butter & sugar; cream well. Add vanilla & combine. In a small bowl, combine dry ingredients; gradually add to creamed mixture. Blend well. Divide dough into 8 portions. Press each portion into a 4 X 3/4-inch mini tart pan. Using a fork, poke some holes in the bottom of each shell. Bake about 10 minutes or until golden brown. Remove from oven & cool before filling.
Blueberries
In a saucepan, whisk all ingredients together & cook over medium-high heat until thickened.
Candied Lemon Curls
Cut the ends off of the lemon. Carefully cut down ONE side of the lemon. Continue the same cut through the FRUIT of the lemon, stopping at the peel. Do NOT cut through the peel.
Carefully open up the lemon & make more cuts through the FRUIT so that it will lay flat; remove the fruit from the peel. Turn the peel over & trim the edges & carefully remove all of the white pith from the inside of the peel. Cut the peel into strips about 1/8-inch wide.
In a small saucepan, add sugar & water & bring to a simmer. Add peels & gently simmer for 10 minutes. After 10 minutes, lay peels on a cooling rack. Allow to cool slightly, then toss in a bit of granulated sugar. They will slightly curl as they cool.
Assembly
Divide custard between tart shells, top with blueberries & garnish with candied lemon curls.
Recipe Notes
I find this recipe works the best if everything is made a day earlier than needed. That way each component has a chance to cool well before you assemble & serve.