HEAT oven to 350°F.
BEAT first 5 ingredients, 1 cup sour cream and 1/2 cup water in large bowl with mixer until blended. Remove 1/2 cup batter; place in small bowl. Spoon remaining batter into 24 paper-lined muffin cups.
MICROWAVE 1 cup caramel bits and 1 Tbsp. (3 tsp.) of the remaining water in microwaveable bowl on HIGH 30 sec.; stir. Continue microwaving until caramels are completely melted, stirring every 10 sec. Add to reserved cake batter; mix well. Stir in remaining sour cream. Drop by spoonfuls over batter in muffin cups; swirl gently with knife.
BAKE 24 to 28 min. or until toothpick inserted in centers comes out clean. Cool in pans 10 min. Remove to wire racks; cool completely.
MICROWAVE remaining caramel bits and water in microwaveable bowl 30 sec.; stir. Continue microwaving until caramels are completely melted, stirring every 10 sec. Cool slightly.
DRIZZLE caramel mixture over cupcakes; sprinkle with nuts. Let stand until caramel is firm.