Make the Maple Cupcakes: Preheat the oven to 325 degrees F. Line two 12-cup cupcake pans with paper liners.
In a medium bowl, sift together the flour, baking powder, and salt.
In the bowl of a standing mixer fitted with the paddle attachment, beat the butter and shortening until ribbonlike. Turn the mixer to low and stream in the maple syrup. Increase the speed to medium-high and beat until the mixture is nearly uniform in color, about 3 minutes.
Add the egg yolks and egg, one at a time, and beat until just incorporated. Scrape down the sides and bottom of the bowl. Add half of the flour mixture and mix on low speed until incorporated. Stream in the milk. Stop the mixer, add the rest of the flour, then turn the mixer on until just combined. Scrape down the sides and bottom of the bowl and fold in the walnuts.
Fill the prepared cupcake pan about three-quarters full. Bake the cupcakes for 20 to 25 minutes, rotating the pans halfway through the baking time, until a toothpick inserted in the center of a cupcake comes out clean. Note: These cupcakes take longer to bake than traditional cupcakes due to the maple syrup.
Allow the cupcakes to cool for 15 minutes in the cupcake pan, then turn them out onto wire racks to cool completely.
Make the Cream Cheese Maple Frosting: In the bowl of a standing mixer fitted with the paddle attachment, beat the softened butter until it is completely smooth. Add the cream cheese and beat until combined.
Add the sugar and the maple syrup and beat until smooth. Be careful not to overbeat the frosting or it will lose structure. (At this point, if you want to, you can tightly cover the frosting and refrigerate it for a day. Let it soften at room temperature before using.).
Assemble the Cupcakes: There are many ways to frost a cupcake. If you have a pastry bag, simply fit with the largest tip, fill the bag with frosting, and pipe enough to cover the cupcake in a big mound. If you do not have a pastry bag, use an ice cream scoop with a release mechanism to scoop the frosting and dispense it onto the top of the cupcake. You can also use an offset spatula to frost the cupcakes.
Refrigerate any leftover cupcakes in an airtight container for up to 3 days. Bring cupcakes to room temperature before serving.