Line a 9x13 pan with parchment paper or foil. Spray the dish with non stick spray.
In a large bowl, whisk together the flour, cocoa powder, sugar, and salt. Using a stand mixer or electric mixer, beat 16 Tablespoons (2 sticks, 1/2 pound) of butter at low speed into the flour until it is incorporated and looks like wet sand. (Alternatively, do this in a large bowl with your fingers, squishing the butter into the flour).
Remove one cup of the flour mixture for the topping and set aside.
Take the remaining flour mixture and press it into an even layer across the bottom of the dish.
Bake the bottom crust in the dish until the edges just start to brown, approximately 15 minutes.
While the bottom crust is baking, combine the reserved flour mixture, oats if using, and brown sugar in a medium bowl. Stir to combine.
Add the remaining 2 Tablespoons of butter to the topping mixture and pinch it to combine, making clumps of the streusel. Set aside.
In a small bowl, combine the raspberry jam and lemon juice. Stir to combine.
When the bottom crust has been removed from the oven, spread the raspberry jam mixture across the warm crust. Then sprinkle the raspberries across the jam, and then the streusel mixture across the top.
Bake the crumble bars for approximately 20-25 minutes until the top is browned and the edges of the apricot mixture are bubbling.
Let cool completely or until just slightly warm, approximately 2 hours. Use the foil or parchment to remove the bars from the dish and cut into pieces. Re-warm if desired in pieces or as a whole. Serve with ice cream or whipped cream if desired.