Begin by preheating your oven to 350 degrees F. Then, line a 9 x 13 baking pan with parchment paper. Spray with baking spray and set aside.
With a stand (or handheld) mixer, cream together the butter and sugar until light and fluffy. Then, beat in the eggs one at a time along with the vanilla extract.
Add the flour, baking powder, and salt. Mix until just combined.
Add a little over half of the cake batter to the parchment-lined baking pan. Use an offset spatula to spread out an even layer. Top with the strawberry rhubarb pie filling and spread out into an even layer.
Finally, add dollops (about 1 tablespoon) of the remaining cake batter over the strawberry rhubarb pie filling. Gently spread into an even layer using your offset spatula. (It's okay if some of the pie filling shows through.)
Bake for 35 minutes or until the top starts to become golden brown. Avoid over baking. (You can test the center of a cake with a toothpick. It should come out clean.) Allow to cool, then slice into rectangles or squares.
For a final finishing touch, drizzle with a confectioners' sugar glaze. To make the glaze, whisk together the sugar, milk, vanilla extract, and salt. (Add the milk a little at a time to achieve the desired consistency.)