Preheat your oven to 350°F (175°C). Line baking sheets with parchment paper.

In a large bowl, cream together the softened butter, brown sugar, and granulated sugar until smooth.

Add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.

In another bowl, whisk together the oats, flour, baking soda, and salt.

Gradually mix the dry ingredients into the wet ingredients until just combined.

Drop tablespoon-sized amounts of dough onto the prepared baking sheets, spacing them about 2 inches apart.

Bake in the preheated oven for 10-12 minutes or until the edges are lightly golden.

Remove from the oven and let cool on the baking sheets for a few minutes before transferring to a wire rack to cool completely.

In a medium bowl, beat the softened butter until creamy.

Gradually add the powdered sugar and mix until smooth and fluffy.

Mix in the maple syrup and vanilla extract until fully incorporated.

Once the cookies are completely cool, spread a generous amount of buttercream on the flat side of one cookie.

Top with another cookie, flat side down, to create a sandwich.

Repeat until all cookies are assembled.