Step 1: In a large bowl, cream together the softened butter, sugar, and sea salt with an electric mixer until light and fluffy (about 2-3 minutes).
Step 2: Beat in the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
Step 3: In a separate bowl, whisk together the all-purpose flour, cornstarch, and optional nutmeg.
Step 4: Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix.
Step 5: Divide the dough into two equal portions. Flatten each portion into a disk, wrap tightly in plastic wrap, and refrigerate for at least 30 minutes, or up to 2 hours.
Step 6: Preheat your oven to 375°F (190°C). Line baking sheets with parchment paper.
Step 7: On a lightly floured surface or between two sheets of parchment paper, roll out one disk of dough to about 1/4-inch thickness.
Step 8: Use fall leaf-shaped cookie cutters to cut out shapes. Carefully transfer the cut-out cookies to the prepared baking sheets, leaving about 1 inch between them.
Step 9: Bake for 10-12 minutes, or until the edges are lightly golden. Let the cookies cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.
Step 10: Repeat with the remaining dough. Once completely cooled, decorate as desired.