Preheat the oven to 350 F. Line two baking sheets with parchment paper.

In a medium bowl, stir together the oats, flour, baking soda, baking powder, cinnamon, and salt.

In a separate bowl, beat the butter and sugar together until light and fluffy. Add the eggs and vanilla and beat until combined.

Add the oat mixture and beat on low speed just until combined.

Use a cookie scoop or spoon to place spoonfuls of dough onto the prepared baking sheets. The cookies will spread a bit, so place them 1 ½ to 2 inches apart on the baking sheets. Use your fingers to slightly flatten the cookies.

Bake for 12-14 minutes, until golden and just set. Let cool for 5 minutes on the pan before transferring to a wire rack to cool completely. The cookies will be a bit crispy but soften with the marshmallow filling.

Place the dark chocolate melting wafers in a disposable piping bag. Heat in the microwave, manipulating the bag every 30 seconds until melted.

Pipe the chocolate into lines onto a wax paper lined baking sheet. Aim to have the lines the length of the diameter of the cookies. Use the offset spatula to lightly smooth the chocolate lines so that the surface is flat.

Place the chocolate lines into the refrigerator to set, about 15 minutes.

Place the white chocolate melting wafers into a disposable piping bag. Heat in the microwave, manipulating the bag every 30 seconds until melted.

Pipe two rows of small dashes onto the dark chocolate lines. Let set.

On medium speed, beat the butter until light and fluffy, about 2 minutes.

Add in the marshmallow fluff and beat on low speed until just combined, about 30 seconds.

Gradually add in the powdered sugar and vanilla extract. Beat on medium speed until fluffy. Make sure not to overbeat it as it will become tough.

Transfer the filling to a piping bag fitted with a large round piping tip.

Flip half of the cookies onto their tops so that their flat side is face up.

Pipe a swirl of marshmallow filling onto the flipped cookies.

Place a second cookie, flat side down, onto the marshmallow filling to make a cookie sandwich.

Pipe a small dollop of marshmallow filling into the center of the tops of the cookie sandwiches. This will hold the chocolate pieces on the cookie.

Gently remove the chocolate decorations from the wax paper and place one, diagonally, onto the top of each cookie sandwich.

Store in an airtight container.