In a bowl, with an electric mixer on medium speed, beat butter and sugar until smooth. Beat in fennel seeds, rum, vanilla, and eggs until well blended, scraping down sides of bowl as needed.
In another bowl, mix flour, cornmeal, and baking powder. Stir or beat into butter mixture until well blended. Stir in pine nuts.
Spoon 1 cup dough in dollops down the length of a buttered or cooking parchment-lined 12- by 15-inch baking sheet, 1 inch from edge. Spoon another 1 cup dough down the length of the other side of the sheet. With floured fingers, pat each strip of dough into a flat 13-inch loaf, about 1/2 inch thick and 2 inches wide. Repeat with remaining dough (dough can stand while first loaves bake).
Bake loaves in a 350° oven until golden, 15 to 20 minutes; if baking more than one pan at a time, switch pan positions halfway through baking.
With a sharp knife, cut loaves crosswise into 1/2-inch-thick slices. Separate slices slightly and tip each onto a cut side.
Return to oven and bake until cookies are slightly darker and firm and dry to the touch, 15 to 20 minutes longer. Gently slide biscotti onto racks to cool completely.
Note: Nutritional analysis is per cookie.