Preheat oven to 350°.
Spray 2 (12-well) mini cheesecake pans with baking spray with flour.
In a medium bowl, stir together cookie crumbs, ¼ cup sugar, melted butter, and salt.
Press mixture into bottoms of wells of prepared pans.
Bake for 10 minutes. Let cool on a wire rack.
Fill a small bowl with ice water. Set aside.
In a small saucepan, bring 2 cups water to a boil over medium-high heat.
Add mint; boil for 10 seconds.
Remove mint using a slotted spoon, and immediately place in ice water; let stand for 30 seconds.
Remove mint from ice water, and squeeze dry with paper towels.
In a small saucepan, bring 1⅔ cups cream and milk to a simmer over medium heat.
In the container of a blender, place cream mixture and mint; blend until mixture turns pale green.
Return mixture to saucepan, and bring just to a boil over medium heat.
In a medium bowl, whisk together egg yolks, cornstarch, and remaining ⅓ cup sugar.
Add ½ cup hot cream mixture to egg mixture, whisking constantly.
Return mixture to pan, and cook, whisking constantly, until thickened, 3 to 5 minutes.
Remove from heat, and transfer to a shallow heatproof bowl.
Press a piece of plastic wrap directly onto surface, and let cool to room temperature.
In a medium bowl, beat mascarpone cheese and remaining 1 cup cream with a mixer at medium-high speed until soft peaks form.
Fold mascarpone mixture into cooled mint custard.
Transfer mixture to a large piping bag with a large hole cut in tip.
Pipe mixture onto prepared crusts, filling wells three-fourths full.
Freeze for at least 2 hours.
Carefully remove tartlets from pans, and let stand at room temperature for 20 minutes before serving.
Garnish with mint, if desired.