Preheat oven to 350° F. Line an 8-inch square metal pan with non-stick foil.
Combine the flour, oats, brown sugar and salt in a large bowl and mix well. Add the softened butter and stir until incorporated. Press in bottom of lined pan and bake for 12 minutes. Let cool.
In a mixing bowl (I used the same one I used for the crust, but wiped away crumbs), mix the granulated sugar, Biscoff spread, pumpkin, evaporated milk, egg, cinnamon and vanilla with a hand-held mixer until creamy. Pour over the crust and bake for about 30 minutes or until top appears set. Let cool completely, then chill thoroughly before serving.