3/4cupunsalted butter(1-1/2 sticks) chilled and cut into small pieces
1-1/2cupsquality jam or fruit preserves
For the crumble topping
1-1/2cupsall purpose flour
1/2cuplight brown sugar, lightly-packed
1/2cuprolled oats(not quick-cooking)
3/4cupunsalted butter(1-1/2 sticks), chilled and cut into small pieces
powdered sugar for dusting(optional)
Preheat oven to 375 degrees F with rack in middle position. Line a 13x9x2-inch baking pan with parchment paper with a 2-inch overhang on the long sides. Lightly coat parchment with cooking spray.
Make the base
In a large bowl, whisk together flour, sugar, kosher salt, and baking powder. Using a pastry blender, a fork, or two knives, cut butter into flour mixture until it appears sandy and forms fine crumbs. (See Notes for food processor instructions.)
In a small measuring cup, stir together milk and almond extract. Pour over the flour mixture and combine until moistened throughout. Mixture will remain crumbly. For a tender shortbread base, don't overwork dough.
Press dough into the prepared baking pan in an even layer. Bake 12-15 minutes, until center is just set and edges are just beginning to turn light brown. Remove from oven to assemble bars and leave the oven on.
Make the oat crumble topping
While the base is baking, whisk together flour, light brown and granulated sugars, salt, baking powder, cinnamon, cardamom (if using), and oats. Using the same process as you did for the crust, cut butter into the flour mixture until crumbly.
Assemble the bars
Spread preserves over hot crust layer. Sprinkle crumble topping in an even layer over preserves (the crumble layer will be thick).
Return to oven and bake 25-30 minutes, until crumble topping is light golden brown. Let stand at room temperature in the baking pan until completely cooled.
Dust with powdered sugar, slice into bars, and serve.
*The brand of ground cardamom I use is very fragrant, so 1/8 teaspoon gives the crumble topping a nice, but not overly-assertive hint of the spice. If your cardamom isn't as aromatic, or if you prefer a stronger flavor, increase to 1/4 teaspoon.**You can also use a food processor to cut the butter into the flour-sugar mixture. For a tender crust and streusel, use short pulses and be careful not to overprocess. When using a food processor, I prefer to incorporate the liquid ingredients in the base and the oats in the crumble topping by hand.