Prep and Cook Time: about 1 1/2 hours. Notes: Oatmeal gives these jam-filled cookies a hearty texture. Store the cookies airtight for up to 2 days.
1 cup (1/2 lb.) butter, at room temperature
1/2 cup firmly packed brown sugar
1 large egg, separated
1 1/2 teaspoons vanilla
1 cup all-purpose flour
2 cups rolled oats
1 1/2 cups finely chopped walnuts
About 1 cup fruit jam
How to Make It
In a bowl, with an electric mixer on high speed, beat butter, brown sugar, egg yolk, and vanilla until smooth. Stir or beat in flour and oats until well blended.
In a small bowl, beat egg white and 2 teaspoons water to blend. Put walnuts in a shallow bowl. Shape dough into 2-teaspoon-size balls and dip in egg white mixture; shake off excess, then roll in walnuts. Place 1 inch apart on buttered or cooking parchment-lined 12- by 15-inch baking sheets. Press your thumb into the center of each cookie to make a 1/2-inch-deep indentation.
Bake cookies in a 300° oven until lightly browned, about 15 minutes; if baking more than one pan at a time, switch pan positions halfway through baking.
Carefully fill each indentation with about 1 teaspoon jam (if indentations have disappeared, make them again with the rounded end of a wooden spoon). Return to oven and bake until cookies are golden brown and jam has melted, about 15 minutes longer. With a wide spatula, transfer cookies to racks to cool completely.