Preheat the oven to 375 F (190 C). Line two large baking sheets with parchment paper.
In a small mixing bowl, whisk the granulated sugar and ground cinnamon together until well combined.
In mixing bowl, whisk the flour, baking powder, cornstarch, ground cinnamon and salt together until well combined.
In a large mixing bowl or stand mixer, beat the butter and sugar together on medium speed until light and fluffy. This takes about 2-3 minutes. Scrape the sides and bowl as needed. With the mixer on low, beat in the egg, yolk and vanilla extract one at a time until fully combined. Scrape the sides and bowl.
With the mixer on low or by hand, mix in the dry ingredients until JUST combined. Scrape the sides and bottom of the bowl. If needed, mix the dough together briefly to incorporate any stray dough or wet ingredients.
Using a medium cookie scooper or spoon, scoop about 2 tablespoons (30 ml) of cookie dough. Roll into a ball and roll into the cinnamon sugar mixture. Place the cinnamon sugar coated balls of snickerdoodle cookie dough onto the lined baking sheets about 3 inches apart.
Bake the snickerdoodles for about 8-10 minutes. The edges will be set and lightly golden brown. The centers will look slightly puffed with little cracks across the top. Cool the snickerdoodle cookies on the hot baking sheet for 5 minutes before transferring them to a cooling rack.