• Preheat the oven to 350°F. Line baking sheets with parchment paper and set aside.

Make the cookie dough.

• In the bowl of a stand mixer (or in a large bowl using an electric hand mixer), beat butter, brown sugar, and sugar until creamy.
• Add egg and vanilla and beat until combined.
• Add flour, baking soda, cinnamon, and salt and mix until combined.
• Add oats and mix just until blended.
• Scoop tablespoons of dough and gently roll each into a ball.

Bake the cookies.

• Place cookie dough balls on a cookie sheet and bake 10 minutes or until cookies are golden brown. Let cookies cool on the tray for a couple of minutes, and then transfer to a wire rack to cool completely.

Make the buttercream filling.

• In the bowl of a stand mixer using the paddle attachment (or in a large bowl using a handheld electric mixer), beat butter on medium-high speed until smooth. 
• Add vanilla and salt and beat on medium-high speed until blended.
• Add sugar slowly – one cup at a time – beating well on medium speed and scraping the bowl often. When all sugar is added the frosting will be slightly dry. 
• Add milk and beat on high speed until the frosting is light and fluffy.
• Add the berry jam and beat until well blended.

Assemble the sandwich cookies.

• Once the cookies are completely cool, spread half of the cookies with a thick layer of buttercream and top with the remaining cookies.