Preheat your oven to 350°F (175°C). This ensures that the cookies bake evenly and rise properly.

In a large mixing bowl, cream together the softened butter, brown sugar, and granulated sugar using an electric mixer on medium speed until light and fluffy, about 3-5 minutes. This incorporates air into the mixture, resulting in a tender cookie.

Add the eggs one at a time, mixing well after each addition. Then, add the vanilla extract and mix until combined.

In a separate bowl, whisk together the flour, baking soda, salt, and cinnamon. Gradually add this dry mixture to the wet ingredients, mixing on low speed until just combined. Overmixing can lead to tough cookies.

Fold in the oats and raisins (if using) with a spatula until evenly distributed.

Drop rounded tablespoons of dough onto a baking sheet lined with parchment paper, spacing them about 2 inches apart to allow for spreading.

Bake in the preheated oven for 10-12 minutes, or until the edges are lightly golden. The centers may look slightly underbaked; they will firm up as they cool.

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

In a mixing bowl, beat the softened butter until creamy. This should take about 2 minutes.

Gradually add the sifted powdered sugar, mixing on low speed until combined. This prevents a sugar cloud from forming.

Add the vanilla extract and milk, mixing until the filling is smooth and fluffy. Adjust the milk to reach your desired consistency.

Take one cooled oatmeal cookie and spread a generous amount of cream filling on the flat side.

Top with another cookie, flat side down, to create a sandwich. Press gently to spread the filling to the edges.

Repeat with the remaining cookies and filling. Store in an airtight container at room temperature for up to 5 days.