Preheat your oven to 350°F (175°C). This ensures that the cookies bake evenly and 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 2-3 minutes.

Add the eggs one at a time, mixing well after each addition, followed by the vanilla extract. This helps to incorporate air into the mixture, making the cookies light.

In a separate bowl, whisk together the flour, baking soda, salt, and cinnamon. This step combines the dry ingredients and ensures even distribution.

Gradually add the dry mixture to the wet mixture, 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.

Using a cookie scoop, drop dough onto baking sheets lined with parchment paper, 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; 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 wire rack to cool completely.

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

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

Add the vanilla extract and milk, then beat on medium speed until the filling is smooth and fluffy. Adjust the milk as needed for desired consistency.

Once the cookies are completely cooled, spread a generous amount of cream filling on the flat side of one cookie.

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 the assembled oatmeal cream pies in an airtight container at room temperature for up to 3 days, or refrigerate for longer freshness.