Preheat the oven to 375°F (190°C). Line a 9 x 9-inch square pan with parchment paper, leaving overhang on two sides for easy lifting. Lightly spray the parchment with nonstick spray.

In a large bowl, whisk together the oats, flour, granulated sugar, brown sugar, baking powder, cinnamon, and salt until evenly combined.

Pour the melted butter and vanilla into the dry mixture. Stir with a sturdy spoon until the mixture becomes crumbly and holds together when pressed.

Reserve about half the mixture for the topping. Press the other half firmly and evenly into the bottom of the prepared pan. Bake for 10–12 minutes, or until the edges are just turning golden.

While the crust bakes, chop the plums into 1-inch pieces and toss them with 1 tablespoon cornstarch so juices will thicken while baking.

Spread the cornstarch-coated plums over the warm, partially baked crust in an even layer. Crumble the remaining oat mixture evenly over the plums.

Return the pan to the oven and bake for 30–40 minutes, until the topping is light golden brown and juices bubble slightly at the edges.

Remove from oven and cool completely on a rack. Once cooled, refrigerate the pan for at least 1 hour to firm up for clean slicing.

Use the parchment overhang to lift the slab from the pan and cut into squares with a sharp knife.