Preheat oven to 325°F and set a rack in the middle of the oven. Line 2 12-cup cupcake pans with paper liners.

Sift together flour, baking powder and salt into a medium bowl and set aside. Add butter and shortening to the bowl of a stand mixer fitted with the paddle attachment. Beat on medium speed until well-combined and fluffy (1 - 2 minutes). Turn the mixer to low, and slowly pour in maple syrup. Increase speed to medium-high and beat for 3 minutes. The mixture will look a bit chunky - that's ok.

Reduce the speed to low and add the eggs (yolks and whole) one at a time. Wait until the egg is fully mixed in before adding the next. Turn the mixer off, scrape down the sides and add half of the flour mixture. Beat on low until just incorporated. Stream in all of the milk, then turn the mixer off again. Add the remaining flour mixture, and beat on low until just combined. Don't over-mix here!

Take the bowl from the mixer, scrape down the sides, and gently fold in chopped walnuts. Scoop the batter into the prepared cupcake liners. You'll want to fill them 3/4 of the way full. Bake for 20 to 25 minutes (one pan at a time) until an inserted toothpick comes out clean. Let the cupcakes cool for 15 minutes before frosting.

To make the frosting, combine egg whites, sugar and cream of tartar in the bowl of a stand mixer. Set the bowl over a medium saucepan filled with simmering water. The water should not touch the bottom of the bowl. Whisk until the sugar dissolves into the egg whites and the mixture looks frothy ( 3 to 4 minutes).

Transfer the bowl a stand mixer fitted with the whisk attachment. Beat for 5 - 7 minutes until stiff, glossy peaks form. You'll want to start on a low speed and gradually work your way to high. Add vanilla extract and whisk until fully incorporated (just a few moments more).

Using a piping bag, swirl the frosting on to the cooled cupcakes. Fire up a blow torch and lightly caramelize the swirls. Store cupcakes covered at room temperature for up to 3 days.