In a large bowl, whisk together cake flour, baking powder, salt, nutmeg, and cinnamon. Set aside.
In the bowl of stand mixer fitted with a paddle attachment (or using an electric hand mixer), cream together sugar, vegetable shortening, egg, and egg yolk. Scrape down bowl as needed to ensure thorough mixing.
Add sour cream. Mix on medium speed until well combined.
Add flour mixture in three additions. Mix on low speed until there are no longer dry streaks of flour. The dough will be VERY tacky and sticky. Use a sturdy spatula or bowl scraper to transfer dough to a medium bowl. Directly cover dough with plastic wrap. Chill dough in the fridge for at least 1 hour, or up to overnight.