Yields 2 dozen
Adapted from America’s Test Kitchen
7 1/2 Ounce(wt)s flour, all-purpose
1 1/4 Cup oats, quick cooking
2 3/10 Ounce(wt)s sugar, white
1/3 Cup sugar, light brown, packed
1/4 tsp baking soda
1/4 tsp salt, table
1/2 Cup pecans or almonds finely chopped
3/4 Cups butter, unsalted cut in pieces, softened
1 Cup preserves, blackberry (seedless)
Adjust oven rack to lower-middle position and heat oven to 350 degrees. Spray 9-inch-square baking pan with nonstick cooking spray. Fold two 16-inch pieces of foil lengthwise to measure 8 inches wide. Fit one sheet in bottom of greased pan, pushing it into corners and up sides of pan (overhang will help in removal of baked squares). Fit second sheet in pan in same manner, perpendicular to first sheet. Spray foil with nonstick cooking spray.
In bowl of standing mixer, mix flour, oats, sugars, baking soda, salt, and nuts at low speed until combined, about 30 seconds. With mixer running at low speed, add butter pieces; continue to beat until mixture is well-blended and resembles wet sand, about 2 minutes.
Transfer 2/3 of mixture to prepared pan and use hands to press crumbs evenly into bottom. Bake until starting to brown, about 20 minutes. Using rubber spatula, spread preserves evenly over hot bottom crust; sprinkle remaining oat/nut mixture evenly over preserves.
Bake until preserves bubble around edges and top is golden brown, about 30 minutes, rotating pan from front to back halfway through baking time. Cool on wire rack to room temperature, about 1 1/2 hours, then remove from pan using foil handles. Cut into 1 1/4- to 1 1/2-inch squares and serve.