In a large bowl, whisk together the all-purpose flour, cornstarch, baking soda, and kosher salt. Set aside.

In a separate large bowl, using an electric mixer, cream together the 10 tbsp unsalted butter and granulated sugar until light and fluffy, about 2-3 minutes. Beat in the vegetable oil until combined.

Add the large egg and egg yolk, and vanilla extract. Beat until just combined, scraping down the sides of the bowl as needed.

Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix. The dough will be soft.

Divide the dough in half, wrap each half in plastic wrap, and flatten into a disk. Refrigerate for at least 30 minutes, or up to 2 days.

Preheat oven to 350°F (175°C). Line baking sheets with parchment paper. On a lightly floured surface, roll out one disk of dough to about 1/4-inch thickness. Cut out shapes using cookie cutters. Place cookies 1 inch apart on prepared baking sheets.

Bake for 8-10 minutes, or until the edges are lightly golden and the centers are set. Let cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely. Repeat with remaining dough.

For the frosting: In a large bowl, using an electric mixer, beat the 3/4 cup unsalted butter until creamy. Gradually add 4 cups (480g) powdered sugar, 1/4 cup (60ml) milk, and 1 tsp vanilla extract. Beat until smooth and fluffy. Add food coloring if desired. Once cookies are completely cool, frost them as desired.