Preheat oven to 375°F. Line a 13x9 baking pan with parchment paper hanging at least 2 inches over the long side.
In a large bowl, whisk together the flour, sugar, baking powder and salt. Use two knives or a pastry cutter to cut the butter into the flour mixture until crumbs form.
Add the milk and vanilla extract and mix until moistened throughout. It should remain crumbly. Press dough into the baking pan in an even layer. Bake for 12-15 minutes, or until the edges are light brown and the center is just set. Remove and set aside.
While you are baking the base, make the crumble topping by whisking together the flour, oats, sugar, baking powder, cinnamon, salt and nutmeg. Use two knives or a pastry blender to cut the butter into the crumble mixture until crumbs form.
Evenly spread the jam onto the base. Sprinkle crumble topping evenly over top.
Bake for 35-40 minutes or until the crumble topping is light golden brown. Let cool completely before cutting into bars.