Preheat oven to 350 degrees F. Grease a 9-inch square baking pan with nonstick spray.
Combine the rhubarb, sugar, honey, lemon juice, thyme, cinnamon, ginger, salt and vanilla bean paste. Allow to sit at room temperature for about 1 hour.
Prepare the cake mix according to the box instructions. Place the rhubarb mixture into the bottom of the pan, reserving half of the juices and adding the other half to the pan. Spread rhubarb and juices evenly, then pour cake batter over the rhubarb.
Bake until golden, 50 minutes to 1 hour. Let cool at least 10 minutes, then invert onto a cutting board. Serve warm with the whipped cream and reserved rhubarb liquid.