Preheat oven to 350°F (177°C).
Lightly grease the bottom and sides of a 10-inch cast iron pan or springform pan (see Chef’s Tips) and place a parchment paper round on the bottom to ensure easy removal. Place prepared pan into the fridge until needed.
Whisk together the all-purpose flour, cornstarch, baking powder, and baking soda in a medium bowl until no lumps remain.
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter with the Redpath® Golden Yellow Sugar and the Redpath® Granulated Sugar until pale in colour and fluffy; 3 minutes.
Add in the egg, and mix on low speed until fully incorporated. Repeat with the egg yolk. Scrape down the sides of the bowl and add in the vanilla extract, salt, and almond extract (if using). Blend until well combined.
Add about half of the dry mixture to the bowl on low speed until just combined. Add the remaining flour. Mix until it forms a dough; do not overmix.
Transfer dough into the chilled prepared pan. Press and smooth the dough evenly into the pan with the bottom of a dry measuring cup dipped in water to prevent sticking.
Place pan into the preheated oven and bake for 25 to 30 minutes, or until the edges are just beginning to turn golden brown (the rest of the cookie should be pale in colour), and a skewer inserted into the centre should come out clean.
Remove from oven and place pan onto a wire cooling rack to cool; 20 minutes.
Carefully run a small offset spatula around the cookie. Remove cookie from the pan using a large offset spatula or, if using a springform pan, unlatch the sides and remove the form.
Carefully place cookie onto a large cutting board. With a sharp knife dipped in hot water and wiped dry with a clean kitchen towel, carefully cut cookie into 8 even wedges. Dip and dry the knife after each cut. Ensure cookie is completely cooled before decorating.
Alternatively, after removing the skillet cookie from the pan, let the cookie cool completely, and decorate as desired. Allow the icing to set slightly; about 2 hours. The icing should still be soft but not runny before carefully cutting the cookie into wedges. This ensures cleaner cuts and prevents the icing from cracking.