Preheat the oven to 350°F. Line a baking sheet with parchment paper.
Find a round cookie cutter or small bowl about 4 inches in diameter to use for scooting the cookies.
Cut the cold butter into cubes.
Cream the cold butter, brown sugar, and granulated sugar on medium speed until a paste forms, about 2-3 minutes.
Add the cold egg, extra yolk, and vanilla. Mix until combined, about 20 seconds. Scrape down the bowl.
In a separate bowl, whisk together the flour, baking soda, cornstarch, and salt until evenly blended.
Add the dry ingredients to the butter mixture. Mix on low speed until almost combined. Scrape down the bowl.
Fold in the chocolate chips, butterscotch chips, and cashews until just combined, making sure everything is distributed evenly. Do not over mix.
Portion the dough into 15 dough balls, place 6 dough balls at a time on a parchment lined baking sheet.
Bake for 10-11 minutes on the center rack of the oven, until the cookies are golden at the edges and soft in the center, they may look underdone but they will continue baking on the sheet on the counter. Do not over bake.
To scoot a cookie into a perfect circle, grab a round object like a bowl or large cookie cutter slightly larger than your cookie. While the cookies are still warm and soft on the baking sheet, place the bowl or cutter over a cookie and gently move it in circular motions to reshape the edges. This quick trick smooths out uneven edges, giving you perfectly round cookies every time!
See storage in notes.