In a large bowl, cream butter and brown sugar until light and fluffy.
Beat in egg yolks.
Combine flour and cocoa; gradually add to creamed mixture and mix well.
Cover and refrigerate for 1-2 hours or until easy to handle.
In a shallow bowl, whisk egg whites until foamy. Place nuts in another shallow bowl.
Shape dough into 1-in. balls. Dip in egg whites, then roll in nuts.
Using a wooden spoon handle, make an indentation in center of each cookie.
Place 1 in. apart on greased baking sheets. Bake at 350° for 8-10 minutes or until set.
Remove to wire racks to cool.
In a microwave, melt white chocolate and butter; stir until smooth.
Spoon about 1/2 teaspoon into each cookie. Top each with about 1/4 teaspoon jam. Store in an airtight container.