½cup(1 stick or 113 g) unsalted butter, , chilled and cut into small pieces
Preheat oven to 375° F with rack in middle position. Line a 9x9-inch baking pan with parchment paper with a 2-inch overhang on the long sides. Lightly coat parchment with nonstick cooking spray.
In a large bowl, whisk together flour, sugar, salt, and baking powder. Using a pastry blender, two forks, or your fingertips, cut butter into flour mixture until it appears sandy (fine crumbs).
In a small bowl, combine milk and vanilla extract. Pour over the flour mixture and use a rubber spatula or wooden spoon to 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.
While the base is baking, whisk together flour, sugars, salt, baking powder, cinnamon, and oats. Using the same process as you did for the crust, cut butter into the flour mixture until crumbly.
Assemble the bars
Spread Bleuberet Blood Orange Jam over hot shortbread crust layer. Sprinkle crumble topping in an even layer over preserves (the crumble layer will be thick). Return to oven and bake 20-25 minutes, until crumble topping is light golden brown.
Let stand at room temperature in the baking pan until completely cooled. Slice into squares and serve. Store bars in an airtight container at room temperature.