1Heat oven to 350°F (325°F for dark or nonstick pan). In 13x9-inch pan, melt butter in oven. Remove pan from oven; turn pan so butter evenly coats bottom of pan. Spread rhubarb evenly in pan. Drizzle with honey and granulated sugar.
2In large bowl, beat cake mix, water, oil and eggs with electric mixer on low speed 30 seconds, then on medium speed 2 minutes, scraping bowl occasionally. Pour over rhubarb mixture.
3Bake 38 to 46 minutes (40 to 50 minutes for dark or nonstick pan) or until toothpick inserted in center comes out clean. Cool at least 30 minutes to serve warm, or cool completely, about 1 hour.
4In chilled medium bowl, beat whipping cream, powdered sugar and vanilla on high speed until soft peaks form. Serve warm cake with a dollop of whipped cream, or frost completely cooled cake with whipped cream. Store loosely covered in refrigerator.