In a small bowl, mix peanut butter and powdered sugar. Drop heaping teaspoons onto a parchment lined plate or baking sheet and freeze for an hour until hard.
In a heavy bottom sauce pan, melt butter and let cool slightly.
Whisk together flour, salt, baking soda and cornstarch.
In a stand mixer fitted with the paddle attachment, cream melted butter, brown sugar and granulated sugar until well combined. About 2 minutes.
Add in whole egg and egg yolk and vanilla extract and mix until thoroughly combined.
Gradually stir in dry ingredients.
Stir in almost all of the chocolate chips. I like to save about ¼ cup to press into the tops of the cookies before baking.
Chill dough for at least one hour before baking.
Scoop out a heaping tablespoon of cookie dough onto a parchment lined baking sheet and make a depression in the center. Place on of the peanut butter fillings in the depression and cover with another heaping tablespoon of dough. Form into a mound, keeping it tall so the cookies will be thick and chewy. Bake six at a time on each sheet as these cookies are large.
When you're ready to bake, preheat oven to 375 degrees.
Bake for 12-15 minutes until cookies are light golden brown. Start checking them at around 9 minutes and rotate the pan if necessary.