Preheat your oven to 350°F (175°C). Prepare a baking sheet by lining it with parchment paper or greasing it lightly.

In a medium mixing bowl, combine the all-purpose flour, chopped walnuts, and salt. Whisk the ingredients until evenly distributed.

In a separate large mixing bowl, use an electric mixer to cream together the butter and brown sugar until the mixture becomes light and airy.

Add the egg and vanilla extract to the butter mixture. Beat thoroughly until well combined. Gradually incorporate the dry flour mixture into the wet ingredients, mixing until a cohesive dough forms.

Scoop small portions of dough and roll them into 1-inch balls. Arrange the dough balls on the prepared baking sheet, ensuring sufficient space between each one. Gently press the center of each ball with your thumb to create a small indentation.

Place the baking sheet in the preheated oven and bake for 12-15 minutes, or until the edges of the cookies turn golden brown. Once baked, transfer the cookies to a cooling rack.

While the cookies are still warm, fill the indentations with your choice of chocolate candy, jam, jelly, or nut pieces. Allow the cookies to cool completely at room temperature before serving. Enjoy these delightful treats!