18 servings pieces
Use the best-quality jam and, if possible, high-fat butter. Substitute your favourite jam flavour for the raspberry, but if it is particularly chunky, chop or blend it so the pieces are small and spread easily.
MAKE AHEAD: The shortbread base can be refrigerated a day in advance. The bars will keep, layered between wax paper in a tightly closed container at room temperature, for 5 days.
For the base and filling
12 tablespoons (1 1/2 sticks) unsalted butter, at room temperature (see headnote)
3/4 cup sugar
2 large egg yolks
1/2 teaspoon vanilla extract
3 cups flour
1/4 teaspoon kosher salt
225g raspberry jam (see headnote)
For the topping
1 cup homemade or store-bought granola (see headnote)
1/2 cup flour
2 tablespoons unsalted butter
Pinch kosher salt
For the base and filling: Line a 33 by 23cm pan with parchment paper, with enough to hang over the two short edges (for ease of lifting out the slab later).
Combine the butter and sugar in the bowl of a stand mixer or handheld electric mixer; beat on medium speed for a few minutes until lightened and fluffy. Add the yolks one at a time, beating until fully incorporated, and then the vanilla extract. Stop to scrape down the bowl.
Add the flour and salt; beat on low speed to form a crumbly dough. Transfer to the pan; use the bottom of a dry measuring cup or glass to press it in, forming a uniform base of dough that's about 3/4-inch thick. Refrigerate for 15 minutes; this will make it easier to spread the jam on top.
Meanwhile, preheat the oven to 165 degrees.
For the topping: Combine the granola, flour, butter and salt in a medium bowl. Use your fingertips to rub the butter into the dry ingredients to make a crumbly topping.
To complete assembly, spread the jam gently over the chilled base. Crumble the topping evenly across the jam. Bake (middle rack) for 25 to 30 minutes, until pale golden on the edges.
Cool completely before using the parchment paper to lift the entire slab out of the pan. Cut into 18 bars of equal size.