Preheat your oven to 350°F (175°C). Grease your cake pan or baking dish to prevent sticking.
In a large mixing bowl, whisk together the flour, baking powder, salt, and sugar to combine dry ingredients evenly.
Add the melted butter, vanilla extract, and stir until the mixture looks crumbly and resembles coarse sand.
Gently fold in the sliced strawberries and chopped rhubarb, making sure the fruit is coated with the flour mixture.
In a saucepan, combine the rhubarb, strawberries, cornstarch, and a splash of water. Cook over medium heat, stirring often, until the fruit releases juices and the mixture thickens to a jam-like consistency. This takes about 8-10 minutes.
Pour the thickened fruit filling into your prepared baking dish, spreading it out into an even layer.
Spoon the batter evenly over the fruit filling, smoothing the top with the back of a spoon or spatula.
Bake the cake in the preheated oven for about 40-45 minutes, until the top is golden brown and a toothpick inserted in the center comes out clean.
Remove from the oven and let it cool slightly before serving. The cake should be tender with bubbling fruit edges and a lightly crisp top.
Serve warm or at room temperature, optionally with a dollop of whipped cream or a dusting of powdered sugar.