Preheat oven to 375°F (190°C). Line two or more baking sheets with foil.

Melt 11 tablespoons salted butter in a medium saucepan over low heat. Remove the saucepan from the heat as soon as the butter is fully melted.

Stir the following into the melted butter until evenly combined: 2 cups quick-cooking oats, 1 cup granulated sugar, 2/3 cup all-purpose flour, 1/4 cup light corn syrup, 1/4 cup whole milk, 1 teaspoon vanilla extract, and 1/4 teaspoon salt.

Use a teaspoon to portion the batter onto the prepared baking sheets, spacing each mound about 3 inches apart.

Use a rubber spatula (or the back of a spoon) to flatten each mound slightly into a thin round.

Bake in the preheated oven 6 to 8 minutes, or until the cookies are golden brown.

Remove the baking sheets from the oven and place them on wire racks. Let the cookies sit on the foil-lined sheets until they are mostly set but still slightly warm (this makes them easier to peel). Carefully peel the foil away from the bottoms of the cookies.

Place 1 3/4 cups (1 bag) milk chocolate chips in a medium microwave-safe bowl. Microwave in 20-second intervals, stirring well between each interval, until the chocolate is completely melted and smooth.

Work quickly while the chocolate is fluid: spread a layer of melted chocolate on the flat side of half the cookies, then top each with one of the remaining cookies (flat side down) to form sandwiches. Press gently so the chocolate spreads to the edges.

Let the assembled cookies sit at room temperature until the chocolate firms.