Simmer butter on medium heat until brown and fragrant but not burnt. Set aside to cool. Process walnuts into fine powder. Mix with flour, powdered sugar, egg whites, honey, almond extract, and lemon juice. Fold in the browned butter to form the batter. Add batter to muffin cup tins and put blueberries on top. Bake at 375F for 15-20 minutes. To get a crispier top, leave in for longer time or broil carefully, as desired.