Step 1: Prepare the Shortbread Dough: In a large mixing bowl, cream together the softened butter and sugar until light and fluffy. Gradually add the flour and salt, mixing until just combined. If desired, incorporate vanilla extract. Do not overmix. The dough should be smooth and slightly crumbly.
Step 2: Make the Filling: In a separate bowl, combine the flour, sugar, and melted butter. Add vanilla and any optional spices. Mix well until the mixture resembles coarse crumbs and comes together into a soft, pliable consistency.
Step 3: Assemble the Cookies: Preheat the oven to 375F (190C). Divide the shortbread dough into two equal portions. Roll out each portion to about 1/4-inch thickness on a lightly floured surface. Using a 3-inch round cookie cutter, cut out circles from the dough.
Step 4: Fill the Cookies: Place a small spoonful of the filling (about 1-2 teaspoons) in the center of each shortbread circle. Gently fold the dough over the filling to form a half-moon shape, or place another cookie circle on top to create a filled circle. Press the edges to seal them firmly, and crimp with a fork for a decorative touch.
Step 5: Bake the Cookies: Place the assembled cookies on a baking sheet lined with parchment paper. Brush the tops with egg wash (if using) for a golden shine. Bake for 12-15 minutes, or until the cookies are lightly golden brown.
Step 6: Cool and Serve: Remove the cookies from the oven and let them cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely. Serve and enjoy!
Optional Tips: For a more intense flavor, brown the butter for the filling before mixing. A touch of lemon or orange zest in the shortbread dough can add a bright, citrusy note. If the filling is too dry, add a teaspoon of melted butter at a time until the desired consistency is achieved.