Preheat oven to 350ºF (180ºC).
Grease a 10 to 12-cup bundt cake pan (26cm) with 2 tablespoons shortening2 tablespoons shortening, making sure every nook, cranny and angle is covered. Dust with 2 tablespoons all-purpose flour, shaking off excess. Reserve.
If you can, toast 1 ½ cups walnuts: bake in a 325°F (160°C) oven for about 10 minutes or until lightly toasted and fragrant. Watch them closely as they start to darken quickly at some point. Let them cool down completely before finely grinding them in a food processor.
Sift 2 ¼ cups all-purpose flour, 1 teaspoon baking powder and ½ teaspoon salt into a medium bowl.
Using an electric mixer, beat 1 ¼ cups unsalted butter and 1 ¼ cups sugar in a large bowl until light and fluffy, about 3 minutes.
Add 5 eggs, one at a time, beating well after each addition.
Mix in ½ cup pure maple syrup, ¾ teaspoon maple flavoring (if using), and ½ teaspoon vanilla extract. The batter may look curdled and will smooth out when adding the flour mixture.
Mix in the dry ingredients in 2 parts, using the mixer at the lowest speed.
Fold in the ground walnuts with a spatula and make sure the batter is thoroughly integrated.
Pour the batter into the prepared pan and smooth out top.
Bake about 1 hour, or until it's cracked and a cake tester or toothpick inserted in the center comes out clean.
Transfer the pan to a wire rack and cool the cake for 20 minutes.
Make sure no batter is stuck to the sides of the pan. You can use a smooth-bladed knife to check the sides and around the center. Carefully turn the cake out onto a cooling rack and cool completely before glazing.
Drizzle the glaze and let it drip down the sides.
Place 8 walnut halves on top or sprinkle ground walnuts before the glaze sets. Let it dry before cutting and serving.
Store leftovers covered in plastic wrap for 2 days at room temperature and then refrigerate.