• Preheat your oven to 350 °F.

Upside Down Topping

• Prepare a 13-inch x 9-inch pan by greasing the sides with butter, then pour the 1/4 cup of melted butter into the bottom of the pan.
• Sprinkle the cup of brown sugar evenly over the melted butter.
• Arrange the strawberries on the brown sugar, then sprinkle evenly with the rhubarb. 
• Press the strawberries and rhubarb gently into the brown sugar. 

Cake Batter

• In a large bowl with a hand mixer, or in the bowl of a stand mixer using the paddle attachment, cream your butter and sugars together until combined.
• Beat in the egg and egg yolk until completely combined, then mix in the vanilla.
• Whisk together your dry ingredients in another bowl.
• Alternating between the dry mixture and sour cream, add them into the butter mixture with your mixer on low speed. Once you've added all of the dry mix and sour cream, mix on low speed for another minute to ensure the ingredients are combined.
• Stir in the two cups of chopped rhubarb.  
• Using a large spatula, drop the batter on top of the strawberry/rhubarb mixture in the 9x13 pan. Gently spread the batter out over top of the mixture in an even layer, trying to ensure that the fruit stays on the bottom.
• Bake in the preheated oven until the cake is golden brown, and a toothpick inserted comes out clean.
• The last place to cook properly with this cake is the very center, where the cake meets the fruit. Make sure that you test with a toothpick to the center. When this area is baked, the cake is done! If the top starts to brown too fast, cover lightly with aluminum foil and continue to cook until the toothpick comes out clean.
• Remove and let cool for 20 minutes. Loosen the edges of the cake around the pan by running a butter knife around the edges.
• Get out a platter that is large enough to flip the cake onto. Place the platter on top of the cake pan, then turn the cake pan over to invert the cake onto the platter. The topping will now be on top, and the cake will now be on the bottom!
• Slice and serve.