Preheat oven to 350℉. Prep an 8x8 pan with a little butter or pan spray and fit with parchment paper, leaving an overhang.
Add all dry ingredients to the bowl of a stand mixer fitted with the paddle attachment. Mix on low speed until combined. Add butter and continue to mix on low speed until crumbles have formed.
Pour two-thirds of the crumble into the pan and press into a smooth, even layer. Top with the jam and smooth out. Add the fresh fruit, if using.
Sprinkle the remaining crumble on top of the jam and gently press it down into the jam.
Bake for 35-45 minutes. The jam should be just starting to bubble and the crumble will be a nice golden brown. Allow to cool completely before cutting into bars.