Heat oven to 350 degrees F. Spray large rimmed baking sheet (1/2 sheet pan size) with cooking spray; line with parchment paper. Whisk flour, cocoa, baking soda and salt in small bowl.
Beat butter and sugar in large bowl at medium speed 3 minutes or until light and fluffy. Add egg, beating until combined. Beat in vanilla. At low speed, beat in flour mixture in 3 parts alternately with milk, beginning and ending with flour mixture. Pour into pan; spread evenly with spatula.
Bake 10-12 minutes or until toothpick inserted in center comes out clean. Cool in pan on wire rack 10 minutes. Place in refrigerator; cool 30 minutes or until chilled and firm.
Meanwhile, beat cream and vanilla in large bowl at medium-high speed until stiff peaks form; put whipped cream in another clean bowl. In same bowl that you beat the cream, beat cream cheese at medium speed 2 minutes or until smooth; beat in caramel topping. Stir in candy bars. Gently fold in whipped cream in 3 parts until just combined.
Invert cake onto cutting board or flat surface; gently peel off parchment paper. Cut cake crosswise in half. Place two large sheets plastic wrap crisscross on large baking sheet. Place one cake layer, top-side down, in center of plastic wrap. Spread filling to within 1/2 inch of edges. Top with remaining cake layer, top-side up. Cover with plastic wrap. Freeze 4 hours or overnight, until firm. Cut into bars. (Store, covered, in freezer up to 1 week.)