Preheat the oven to 300℉ and line the bottom and sides of an 8x8" baking pan with parchment paper.
Beat the butter on high speed for 5 minutes until it turns pale, then mix in the brown sugar, vanilla, and salt. Add the flour and mix until combined.
Press the mixture into the parchment-lined 8x8” pan and bake for 15 minutes then cool on a wire rack. Increase the oven temperature to 350℉.
PUMPKIN LAYER: In a large bowl, whisk together the pumpkin puree, sweetened condensed milk, eggs, vanilla, and pumpkin pie spice until smooth. Pour over the cooled base and spread evenly. Bake for 25 minutes.
CRUMBLE TOPPING: Mix the flour, brown sugar, oats, salt, cinnamon, and melted butter with a fork until just combined and crumbly.
Remove the pan from the oven and carefully sprinkle the crumble evenly over the top. Bake for another 15-20 minutes until set with a slight jiggle in the center.
Cool completely in the pan, then transfer to the fridge for at least 3 hours. Cut into 9-12 pieces and serve.
Recipe by our friend Callan @TheCozyPlum