Preheat oven to 350°F.
Combine butter and sugar in stand mixer; mix on speed 2 for 2 minutes.
Add eggs, one at a time, until combined.
Add lemon extract and zest, mixing until blended.
Add baking powder, salt, and flour, mixing until combined.
Add blueberries, stirring until evenly distributed.
Divide dough in half.
Shape each half into a 10″ x 3" rectangle, using floured hands.
Set each loaf 2″ apart on a parchment paper-lined baking sheet.
Bake for 30 minutes or until the dough is set.
Leaving the oven on, remove the biscotti loaves and cool for 15 minutes on the baking sheet.
Using a serrated knife, cut the loaves into diagonal slices, 3/4″ thick.
Place slices on cookie sheet with the cut sides down; bake for 8 to 9 minutes.
Turn over slices, and bake for 8 to 9 minutes more.
Remove biscotti from oven, and allow to cool completely on a cooling rack.
Combine white chocolate and shortening in a small microwave-safe bowl.
Microwave for 30 seconds, then stir.
Continue heating chocolate in 30 second increments, stirring in between, until chocolate glaze is smooth.
Using a spoon, spread a layer of glaze on the top of each biscotti.
Refrigerate for 15 minutes to set glaze.