Line a 9 × 9 in pan with parchment, leaving overhang. Beat butter, sugar, and vanilla until smooth. Stir in oat flour, cornstarch, and salt until no dry spots remain. Press evenly into the pan. Bake at 350°F (175°C) for 10 minutes.

Toss diced apples with cornstarch, brown sugar, cinnamon, and nutmeg until coated.

Microwave 5 minutes, stirring once, until slightly thickened.

Or cook in a saucepan over medium heat until glossy and thick but apples keep their shape.

Combine oats, cornstarch, brown sugar, cinnamon, and salt. Work in softened butter with fingertips until coarse crumbs form that hold when pinched.

Spread warm apple filling over the par-baked crust, reaching the corners. Scatter crumble evenly on top, keeping some larger clumps for texture.

Bake 25 to 30 minutes until the top is golden and the edges show slow bubbles.

Cool completely in the pan on a rack for clean cuts. Lift out by parchment and cut into 9 bars.