While your air fryer preheats to 190°C (375°F), make the crumble topping first so it's ready when you need it. In a bowl, combine the cold cubed butter with the flour, using your fingertips to rub them together until the mixture resembles coarse breadcrumbs with some pea-sized butter pieces still visible. Stir in the brown sugar, ground cinnamon, and salt until evenly distributed. Set aside—I like to keep this mixture cool until assembly, as the cold butter helps create a better texture when air frying.

While the crumble topping rests, prepare the filling by combining the white sugar, flour, and vanilla extract in a separate bowl and mixing well. Add the cut rhubarb pieces and stir gently until all pieces are evenly coated with the sugar mixture. This allows the rhubarb to start releasing its juices and helps it cook evenly in the air fryer.

Lightly spray two individual ramekins (or one 8x6 inch air fryer-safe dish) with oil. Divide the rhubarb filling evenly between the ramekins, then top each with a generous portion of the crumble topping from Step 1, spreading it evenly but leaving some texture for a rustic appearance. Don't press down on the topping—I find that a loosely packed crumble creates better texture as it crisps up in the air fryer.

Place the ramekins in the preheated air fryer and cook at 190°C (375°F) for 15 minutes, or until the crumble topping is golden brown and the rhubarb filling is bubbling slightly at the edges. The rhubarb should be tender but still hold its shape. Let cool for 2-3 minutes before serving—this allows the filling to set slightly while the topping remains crispy.