Adjust an oven rack to the middle of the oven. Preheat the oven to 350F [180C]. Line a 9 by 9 in [23 by 23 cm] square baking pan with a parchment sling.
In a small bowl, whisk together the flour, cream of tartar, and baking soda.
In the bowl of a stand mixer fitted with a paddle, mix the butter and shortening together on medium until creamy, about 1 minute. Scrape down the sides and add the sugar, salt, and nutmeg and beat on medium until light and fluffy, 2 to 3 minutes.
Add the egg, egg yolk, and vanilla, and beat on low speed until combined. Add the flour mixture and beat again on low speed until combined.
Combine the granulated sugar and cinnamon for the topping together in a small bowl.
Pat the dough into an even layer in the prepared pan. Sprinkle the sugar topping evenly over the dough.
Bake the bars for 16 to 20 minutes, until a wooden skewer or toothpick inserted into the center comes out just clean (see notes).
Transfer the pan to a wire rack and let cool completely. Use the parchment sling to remove the bars from the pan, then cut into pieces and serve. Bars can be store in an airtight container at room temperature for up to 3 days.