Preheat oven to 375. Line pan with parchment. Prepare parchment with butter to ensure the bars release easily from the pan. Set aside.
In a medium bowl, mix flour, sugar, baking powder and salt. Set aside.
In a small bowl, whisk egg, lemon zest and vanilla. Combine with dry ingredients and stir until well combined.
Using a pastry cutter, cut in the butter until pea sized pieces form. The mixture will be crumbly.
Press about 3/4 of the mixture into the bottom of the pan until it forms a shortbread like crust on the bottom, reserving enough to sprinkle on the top of the bars. Set aside.
Next, combine corn starch and sugars in a large bowl and stir until well mixed. Add in sliced rhubarb and strawberries with the lemon juice.
Pour strawberry rhubarb mixture over the prepared bottom. Spread the remaining dough mixture over the top of the strawberry rhubarb filling.
Bake for 45-50 minutes or until the bars are a golden brown on the top.
Cool for 15-30 minutes and slice and serve. Store in the refrigerator.