Chewy Black + White Chip Oatmeal Sandwich Cookies with Marshmallow Buttercream Filling

• 1/2 cup unsalted butter at room temperature
• 1/2 cup sugar
• 1/2 cup brown sugar loosely packed
• 1 egg large
• 2 teaspoons vanilla extract
• 1 1/4 cup all-purpose flour
• 1/2 teaspoon baking soda
• 1/4 teaspoon salt
• 3/4 cups rolled oats
• milk if dough is super crumbly, optional, 1-2 tablespoons
• 2/3 cup dark chocolate chips
• 2/3 cup white chocolate chips

marshmallow buttercream filling

• 1/2 cup unsalted butter softened
• 1 cup marshmallow cream homemade or store-bought is fine
• 1/2 cup powdered sugar
• 1 teaspoon vanilla extract
• 1 pinch salt