1Heat oven to 325°F. Grease and flour 12-cup fluted tube cake pan, or spray with baking spray with flour.
2Make cake batter as directed on box--except stir poppy seed into batter. Pour into pan.
3Bake as directed on box for fluted tube pan. Cool in pan 15 minutes; turn upside down onto heatproof serving plate. Remove pan; cool cake completely, about 1 hour.
4In microwavable bowl, microwave frosting uncovered on Medium (50%) 15 seconds. Spread over top of cake, allowing some to drizzle down side. Store loosely covered.