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

In a large mixing bowl, cream together the softened butter, brown sugar, and granulated sugar using an electric mixer on medium speed until the mixture is light and fluffy, about 3-4 minutes.

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

In a separate bowl, whisk together the flour, baking soda, salt, oats, and cinnamon until well combined.

Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix, as this can lead to tough cookies.

Using a tablespoon or cookie scoop, drop rounded tablespoons 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. The centers may look slightly underbaked, but they will firm up as they cool.

Remove from the oven and allow the cookies to cool on the baking sheets for 5 minutes before transferring them to a cooling rack to cool completely.

In a clean mixing bowl, beat the softened butter with an electric mixer until creamy.

Gradually add the sifted powdered sugar, mixing on low speed until combined. Increase the speed to medium and beat until fluffy, about 2-3 minutes.

Add the vanilla extract and heavy cream (or milk) and beat until the filling is smooth and spreadable.

Take one cookie and spread a generous amount of cream filling on the flat side. Top with another cookie, flat side down, to create a sandwich.

Repeat this process with the remaining cookies and filling.

Serve immediately or store in an airtight container in the refrigerator for up to 5 days.