Preheat oven to 350 degrees F (175 degrees C).

Beat sweet butter and cream cheese together in a bowl with an electric mixer until smooth and creamy. Gradually stir flour into butter until forming a dough. Shape dough into 1-inch balls and place into 3/4-inch muffin cups. Press dough into the bottom and sides of each muffin cup.

Mix brown sugar, walnuts, melted butter, eggs, and vanilla extract together in a separate bowl. Spoon about 1 teaspoon of walnut filling into the dough cups.

Bake in the preheated oven until edges of cookies are lightly browned and filling is cooked through, about 25 minutes.