Preheat oven to 350°F Line 18 muffin cups with cupcake liners.
Grate 2 tsp zest and squeeze 3/4 cup juice from oranges. On medium speed, beat cake mix, eggs, orange juice, 1/2 c water, 1/4 c butter and 1 tsp zest until combined (30 secs). On medium-high speed, beat until smooth and thickened (2 mins).
Divide among liners. Bake 18-20 mins or until toothpick inserted into centers comes out clean. Cool 10 minutes Transfer to racks. Cool completely.
On medium speed, beat remaining 1 1/2 c butter until smooth. Gradually beat in sugar until light and fluffy (2 mins). Beat in remaining 1 tsp zest. If desired, tint frosting orange with food colorings, then transfer to pasty bag fitted with star or other decorative tip. Pipe or spread frosting over cupcakes. Garnish with sprinkles.