1. In 4-cup glass measure, combine chocolate chips, sweetened condensed milk and 1 tablespoon butter.
2. Microwave on High for 1 to 2 minutes, stirring after every 30 seconds. Stir until chocolate is melted.
3. In large mixing bowl, stir together brown sugar, 3/4 cup butter, eggs and vanilla until smooth.
4. Stir in Pioneer Baking Mix and oats until well combined.
5. Spoon 2/3 of the batter into a 13 x 9 x 2-inch baking pan that has been coated with cooking spray; press to flatten.
6. Drop chocolate mixture by teaspoonfuls over batter.
7. Drop teaspoonfuls of remaining batter over top of chocolate; swirl together with tip of knife or wooden pick.
8. Bake at 350˚F for 20 to 25 minutes or until bars are light golden brown and center is almost set.
9. Cool completely. Cut into bars.