Preheat your oven to 180°C (350°F). In a mixing bowl, combine the oats, all-purpose flour, light brown sugar, ground cinnamon, and a pinch of salt. Melt the unsalted butter and pour it into the dry ingredients, mixing until crumbly and evenly coated. Press half of this mixture into the bottom of a greased baking pan to form the crust.

Slice the plums and remove pits. In a medium bowl, toss the plums with cornstarch and vanilla extract until well coated. Spread the plum mixture evenly over the crust layer, ensuring a juicy, tart filling. Crumble the remaining oat mixture over the top, creating a rustic, crumbly surface.

Bake in the preheated oven for 40-45 minutes or until the topping is golden brown and the plums are bubbling. Let the bars cool slightly before slicing. For best flavor, serve warm or at room temperature. Enjoy the melty, fruity, sweet-tart goodness that is irresistibly cozy and delicious!