1
Brown the butter: Melt butter in a saucepan over medium heat. Stir constantly until it foams, then turns golden brown with a nutty aroma (about 5–7 minutes). Remove from heat and cool 10–15 minutes.
2
Mix wet ingredients: In a large bowl, combine browned butter, brown sugar, and granulated sugar. Mix well. Add eggs and vanilla and beat until glossy.
3
Add dry ingredients: Stir in flour, baking soda, cinnamon, and salt. Fold in oats last.
4
Chill the dough: Cover and refrigerate for 30–60 minutes (this keeps the cookies thick and chewy).
5
Bake: Preheat oven to 350°F. Scoop dough into ~2 tbsp balls. Place on parchment-lined baking sheet, 2 inches apart. Bake for 10–12 minutes, until edges are golden and centers are soft.
6
Make the filling: Beat softened butter until creamy. Add powdered sugar, cream, vanilla, and salt. Beat until fluffy and smooth, about 2–3 minutes.
7
Assemble: Pipe or spread the filling onto half the cookies. Top with the other half and gently press to sandwich.
8
Chill slightly before serving: Optional, but it helps the filling set and makes them even better.