Prep your tools: Heat the oven to 350°F (175°C). Line two baking sheets with parchment paper. Soften the butter to room temp.

Mix dry ingredients: In a bowl, whisk 1 1/2 cups flour, 1 teaspoon baking soda, 1 teaspoon cinnamon, and 1/2 teaspoon salt. Stir in 2 cups rolled oats. Set aside.

Cream the butter and sugars: In a large bowl, beat 1 cup unsalted butter with 3/4 cup brown sugar and 1/2 cup granulated sugar until fluffy, about 2 minutes.

Add eggs and flavor: Beat in 2 large eggs, 2 teaspoons vanilla, and 1 tablespoon molasses until smooth.

Combine: Add the dry mixture to the wet and mix on low just until no dry streaks remain. Do not overmix.

Chill briefly: Cover and chill the dough for 20–30 minutes. This helps keep the cookies thick and chewy.

Scoop and bake: Scoop 1-tablespoon mounds onto the sheets, leaving 2 inches between. Bake 9–11 minutes, until edges are set and centers look slightly underdone.

Cool completely: Let cookies rest 5 minutes on the sheet, then move to a rack to cool. Pair similar sizes for neat sandwiches.

Make the filling: Beat 1/2 cup unsalted butter (soft) until creamy. Add 1 3/4–2 cups powdered sugar, 1 teaspoon vanilla, a pinch of salt, and 1–2 tablespoons heavy cream until fluffy. For a classic vibe, beat in 1/2–3/4 cup marshmallow creme. Filling should be thick yet spreadable.

Assemble: Pipe or spread 1–2 tablespoons filling on the flat side of one cookie. Top with a second cookie and press gently to spread.

Set and serve: Let the sandwiches rest 15 minutes so the filling firms up. Enjoy!