Make the dough: Finely grind pecans in a food processor; set aside. Put mascarpone and butter into bowl of electric mixer fitted with paddle attachment. Mix on medium-high speed until well blended. Add flour, ground pecans, and salt; mix just until dough comes together.
Roll dough into 16 one-inch balls, and press into bottoms and up sides of cups of mini-muffin tins.
Make the filling: Whisk the egg, sugar, maple syrup, vanilla, butter, and salt in a small bowl. Stir in toasted pecans. Spoon about 1 1/2 teaspoons filling into each muffin cup.
Bake until crust begins to turn golden, about 15 minutes. Let cool completely in tins on wire rack. Store in single layers in airtight containers up to 3 days.