Preheat oven to 350°. Line baking sheets with parchment paper or silicone liners.
Whisk together flour and salt. Stir in pecans. Set aside.
Using an electric mixer on medium speed, beat butter, cream cheese, and sugar until light and fluffy (about 3 minutes). Scrape down sides of bowl. Add vanilla and mix for about 1 minute.
Reduce mixer speed to low and add flour mixture. Mix for 20 seconds. Finish mixing by hand with a rubber spatula.
Scoop out about 1 & 1/2 tablespoons of dough and roll into a ball. Place on prepared baking sheets. Gently press ball. Repeat with remaining cookie dough, leaving an inch between cookies.
Bake each pan 14-16 minutes, or until edges are lightly brown. Transfer cookies to wire racks to cool completely.
Spread dulce de leche on the bottom side of half of the cooled cookies and top with another cookie (bottom sides facing each other).
Makes about 36 cookies, or 18 sandwich cookies.