The other vital component is the filling. Opinions and tastes vary. My wife is partial to lots of filling. I am middle of the road when it comes to this topic but we both agree that the thin, near non-existent layer of filling that some commercial bakeries try to pass off as a proper date crumble these days is really unacceptable. Try this recipe and you'll easily see the difference.
First prepare the filling. In a medium saucepan combine and gently boil until thickened (about 10-15 minutes).
18 ounces dates, chopped
3 tbsp butter
3 tbsp sugar
1 1/2 tbsp vanilla
2 cups boiling water
The filling should be very thick; you will need to watch it carefully and stir constantly in the last few minutes of cooking time. Set aside to cool slightly while you prepare the crumble mixture.
In a large bowl toss together:
2 cups rolled oats (large)
2 tsp baking powder
1 cup unsweetened coconut
2 cups flour
1 cup brown sugar
Using your hands, rub thoroughly through the dry ingredients:
1 ½ cups butter, cut in small cubes.
Divide crumble mixture in half and press half into the bottom of a 9x13 inch greased baking pan. Spread date mixture over the base. Top with remaining crumb mixture, pressing it together in clumps in your hands and then breaking it into small pieces with your fingers. Press down lightly. Bake for 40 minutes at 350 degrees F.
This is a very good basic recipe but I have also had very good variations of date crumbles over the years.
Orange Date Crumbles: Substitute orange juice for the water in the filling and add the zest of a medium orange.
Cocoa Date Crumbles: Add 4 tbsp cocoa to the filling.
Hazelnut or Almond Date Crumbles: Prepare filling as usual. Add ½ cup ground Almonds or Hazelnuts plus 1 tsp cinnamon and ½ tsp nutmeg to the crumble.