Heat oven to 350°F (325°F for dark or nonstick pans). Spray bottoms and sides of 3 (8-inch) round cake pans with cooking spray and line with parchment paper.
In a large bowl, beat Cake ingredients with an electric mixer on medium speed for 2 minutes, scraping the bowl occasionally. Pour 1 2/3 cups of batter in each pan. Spread evenly.
Bake 20 to 23 minutes or until a toothpick inserted in the center comes out clean. Cool 10 minutes; remove from the pans to cooling racks. Cool completely, about 1 hour.
In a small microwavable bowl, microwave chocolate chips and butterscotch chips uncovered on High 60 seconds, stirring halfway through. Continue to microwave in 15-second increments, stirring after each, until chips can be stirred smooth; cool 10 minutes.
In a large bowl, beat 1 1/2 cups softened butter and the melted chips with an electric mixer on medium speed until blended. On low speed, beat in powdered sugar, milk, and vanilla, scraping the side of the bowl occasionally, until smooth and creamy.
Place one cake layer on a serving plate. Spread 2/3 cup of the frosting on top; top with the second cake layer. Spread 2/3 cup of the frosting on top; top with the third cake layer.
Frost the side and top of the cake with remaining frosting. Press crushed pretzels along the bottom edge of the cake. Refrigerate uncovered for at least 1 hour.
When ready to serve, carefully pour and spread caramel sauce over the top of the cake, allowing some to drip down the sides; sprinkle sea salt over the top. Cover and refrigerate any remaining cake.
To soften butter, cut in small pieces, and leave at room temperature. A faster method is to microwave butter in 5-second increments at 50% (Medium) power, turning butter frequently, so butter is softened but not melted.
An easy way to crush pretzels is to place them in a resealable food-storage plastic bag; seal the bag, and crush with a rolling pin.
To keep your serving plate clean while frosting the cake, place strips of waxed paper around the edge of the plate. Remove after frosting is complete.
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