In a large bowl whisk together eggs, melted butter, milk & vanilla. Add flour, sugar, baking powder & salt & mix well to combine. Try to whisk out most of the lumps, but it’s ok if some remain.
Melt 2 tbsp butter in an 8- or 10-inch ovenproof skillet (such as cast iron) over medium low heat. Swirl to cover the bottom & sides of the pan. Add the sugar & cinnamon. Stir. The mixture will look dry at first but will loosen as the sugar melts.
Add the diced rhubarb to the pan and cook, stirring with the sugar until rhubarb starts to become soft. Add diced strawberries & cook for another minute.
Spread the fruit out into an even layer & add the batter mixture. Turn the heat off the pan & move the pan to the oven. Bake for 15 minutes.
Lower the oven temperature to 350 F. & bake for an additional 10 minutes.
Remove from the oven and let sit for 5 minutes. Flip the pancake by running a knife around its edges to loosen it from the pan & placing a plate larger than the pan on top of it. Use an oven mitt & flip in one smooth motion. The pancake should release from the pan easily.