Beat butter, sugar, and vanilla extract together in a bowl until creamy and smooth. Mix flour and baking powder together in a separate bowl; gradually beat flour mixture into creamed butter mixture until dough is smooth. Fold walnuts into the dough. Refrigerate dough for 1 hour.
Preheat oven to 325 degrees F (165 degrees C).
Roll cookie dough into 6 dozen small balls and place them 2 inches apart on a baking sheet. Press the center of each ball using your thumb to form a small well. Fill the depressions with jam.
Bake in the preheated oven until cookies are light golden brown, about 18 minutes.
Melt white chocolate and shortening together in the top of a double boiler over simmering water, stirring frequently and scraping down the sides with a rubber spatula to avoid scorching. Drizzle melted white chocolate mixture over cookies.
Cool cookies on the sheet for 5 minutes before removing to cool completely on a wire rack.