Cream together the softened butter and granulated sugar in a large bowl until light and fluffy.
Beat in the egg and vanilla extract until well combined.
In a separate bowl, whisk together the all-purpose flour and salt.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Cover the dough and refrigerate for at least 30 minutes.
Preheat oven to 350°F (175°C). Line baking sheets with parchment paper.
Roll out the dough on a lightly floured surface to about 1/8-inch thickness. Cut out cookie shapes using a cookie cutter.
Place cookies on the prepared baking sheets, spacing them about 1 inch apart.
Bake for 10-12 minutes, or until the edges are lightly golden.
Let cookies cool on the baking sheets for a few minutes before transferring them to a wire rack to cool completely.
Melt the semi-sweet chocolate chips with coconut oil (if using) in a microwave-safe bowl in 30-second intervals, stirring in between, until smooth.
Once cookies are completely cooled, spread a thin layer of melted chocolate on the flat side of one cookie and top with another cookie to form a sandwich.
Let the chocolate set completely before serving or storing.
Notes
Store cookies in an airtight container at room temperature for up to 3 days.