Press two-thirds of the crumble mixture into the base of the prepared baking pan. Use the back of a spoon to press into a firm, even layer. Set aside the remaining third.
Spread the date filling in an even layer over the base. Add the reserved crumble topping over the date mixture.
Bake for 20-25 minutes, or until the edges and top are a light golden colour. Cool fully before slicing. If possible, freeze for about 10 minutes before slicing for the tidiest bars. Store in the freezer and serve cold.
Date Filling
Add the dates, nut butter, vanilla, and salt to the bowl of a food processor fitted with the blade attachment. Blend on high speed until a very smooth paste forms. If the mixture is too firm to spread, add a tablespoon of water and blend to incorporate. Add a second tablespoon if needed. If the dates are soft enough, no water should be required.
* To make your own oat flour, grind rolled oats in a food processor until a fine flour forms.** Most dates will already have the seeds removed, but it doesn't hurt to check.