1 lb (4 sticks) unsalted butter, at room temperature
1 c firmly-packed light brown sugar
1 c granulated sugar
2 t pure vanilla extract
1 t salt
3¾ c unbleached all-purpose flour
1 c almond flour
1½ c blackberry preserves
½ c slivered almonds
confectioners’ sugar for dusting
Beat the butter and sugars in a large bowl at medium-low speed until creamy. Add vanilla and salt; beat until combined. Add flours one cup at a time, beating on low speed, just until a smooth, soft dough forms. Spray a 9×13″ baking pan with nonstick cooking spray, and press one-third of the dough evenly into the pan to form a bottom crust. Wrap the remaining dough in plastic wrap and refrigerate until very cold and firm, at least 30 minutes. Position a rack in the middle of the oven, and preheat to 325°. Bake the bottom crust until it is firm and just beginning to turn pale brown around the edges, about 20 min.
Remove the pan from the oven and spread the preserves over the crust. Crumble the remaining dough over the jam to form a pebbly, crumbled topping. Sprinkle with almonds. Return the pan to the oven and continue baking until topping is firm and crisp, about 30 mins. Remove from oven and cool to room temperature.
Use a sharp knife to cut bars into 15 large squares, then cut in half on the diagonal to form 30 triangular bars. Remove bars from pan with metal spatula, dust with confectioners sugar (using a sifter or mini-strainer), and serve. Bars will keep, covered tightly, for about a week at room temperature, or up to one month in the freezer.