Ice Cream Pretzel Cake

1-1/4 cups crushed pretzels
6 tablespoons cold butter, cubed
3/4 cup hot fudge ice cream topping, warmed
2 packages (7-1/2 ounces each) chocolate-covered miniature pretzels
1/2 gallon vanilla ice cream, softened
1/4 cup caramel ice cream topping