In a large bowl beat butter with an electric mixer on medium to high for 30 seconds. Add sugar, baking powder, and salt. Beat until combined, scraping sides of bowl occasionally. Beat in eggs and vanilla until combined. Beat in as much of the flour as you can with the mixer. Using a wooden spoon, stir in any remaining flour. Divide dough in half. Cover and chill dough about 1 hour or until easy to handle.
Meanwhile, for filling, in a small saucepan combine blueberry preserves and cornstarch; heat and stir over medium heat until thickened and bubbly. Remove from heat. Stir in walnuts. Cover. Set aside to cool.
Roll half of the dough between two sheets of waxed paper into an 11-inch square. Spread half of the filling over the square, leaving a 1/2-inch border along the edges. Roll up dough into a spiral. Moisten edges; pinch to seal. Wrap log in waxed paper; twist the ends. Repeat with the remaining dough and the remaining filling. Chill for at least 4 hours or up to 3 days.
Preheat oven to 375 degrees F. Line cookie sheets with parchment paper; set aside. Cut chilled logs into 1/4-inch slices. Place slices about 1 inch apart on prepared cookie sheets. Bake for 10 to 12 minutes or just until firm. Cool on cookie sheets for 2 minutes. Transfer to wire racks; cool.