Topping

• Preheat oven to 350 degrees. Coat bottom and sides of a 9 x 13 x 2 inch high baking pan with nonstick cooking spray then set aside.
• In a saucepan over medium heat, melt butter with brown sugar then stir for two minutes until bubbly and caramelized.
• Pour mixture into bottom of prepared baking pan then add chopped rhubarb in an even layer.
• In a large mixing bowl, sift together flour, baking powder, baking soda and salt. Set aside.
• In a separate mixing bowl, beat butter and sugar with an electric mixer until light and creamy, approximately 2 minutes.
• Add eggs one at a time, beating after each until well blended. Scrape down the sides of the mixing bowl with a spatula as needed.
• Incorporate buttermilk and vanilla then beat until combined.
• Reduce mixer speed to low and gradually add flour mixture to wet ingredients. Beat until just combined. Avoid overmixing.
• Spoon batter over rhubarb mixture then smooth top in an even layer.
• Bake for 38 to 43 minutes or until toothpick inserted in the center comes out clean. Do not overbake.
• Remove cake from oven and immediately loosen edges with a knife. Allow cake to cool for 10 minutes then place a large, rectangular serving plate over the top of the cake pan and quickly invert the cake onto the plate.
• Cool for an additional 15 minutes. Slice and serve warm.
• Cool cake completely then cover tightly with plastic wrap and store in the refrigerator up to 3 days.