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.
Recipe by TeaTime Magazine at https://teatimemagazine.com/mint-chocolate-tartlets/