PREHEAT oven to 375° F. Line baking sheets with foil.
MELT butter in medium saucepan; remove from heat. Stir in oats, sugar, flour, corn syrup, milk, vanilla extract and salt; mix well. Drop by level teaspoons, about 3 inches apart, onto foil-lined baking sheets. Spread thinly with rubber spatula.
BAKE for 6 to 8 minutes or until golden brown. Cool on baking sheets on wire racks. Peel foil from cookies.
MICROWAVE morsels in medium, uncovered, microwave-safe bowl on MEDIUM-HIGH (70%) power for 1 minute; STIR. Morsels may retain some of their original shape. If necessary, microwave at additional 10 to 15-second intervals, stirring just until morsels are melted. Spread thin layer of melted chocolate onto flat side of half the cookies. Top with remaining cookies to make sandwiches.