Preheat oven to 350°F (175°C) and line two baking sheets with parchment paper.

In a bowl, cream together softened butter and granulated sugar until fluffy. Add vanilla extract and mix well.

In another bowl, whisk together flour, cocoa powder (if using), and salt. Gradually add this mixture to the butter-sugar blend until fully incorporated.

Scoop tablespoon-sized portions of dough and roll into balls, placing them on prepared baking sheets about 2 inches apart.

Bake for approximately 8 minutes or until edges are lightly golden. Let cool on the pan for 5 minutes before transferring to wire racks.

For the filling, mix powdered sugar with softened butter until fluffy; add flavorings as desired.

Spread filling between two cooled cookies to create delightful sandwich treats.