Step 1

Heat oven to 350°F. In 13x9-inch pan, melt butter in oven. Sprinkle brown sugar and walnuts over butter; stir to combine. Sprinkle chopped apples in pan.

Step 2

Mix cake as directed on box for 13x9-inch pan, using water, oil and eggs. Pour batter over apples and walnuts in pan.

Step 3

Bake 35 to 40 minutes or until toothpick inserted in center comes out clean. Cool on cooling rack 2 minutes; run knife around sides of pan to loosen cake. Place heatproof serving plate upside down on pan; turn plate and pan over. Remove pan. Serve cake warm.