Step 1: Prepare Cookie Dough - Sift together flour and salt in a large bowl, then mix in chopped walnuts for added flavor and texture.
Step 2: Cream Butter and Sugar - Beat softened unsalted butter with powdered sugar until light and fluffy; add vanilla extract and mix in flour-walnut mixture until just incorporated.
Step 3: Chill Dough - Form dough into a disc shape, wrap in plastic wrap, and refrigerate for about 30 minutes.
Step 4: Bake Cookies - Preheat oven to 350°F (175°C), roll out chilled dough to about 1/4-inch thickness, cut out rounds, chill briefly, and bake for 8-10 minutes until lightly browned.
Step 5: Make Ice Cream Base - Combine milk, heavy cream, salt, and dulce de leche in a medium saucepan over medium heat; bring to a gentle simmer while stirring until completely dissolved.
Step 6: Assemble Sandwiches - Cut out discs of frozen ice cream using a cookie cutter and layer between two walnut shortbread cookies, optionally rolling the sides with more chopped walnuts.