In a saucepan, melt the butter over medium heat. Add the peanut butter and stir until melted. Remove from the heat.
Pour the melted butter and peanut butter into a mixing bowl. Stir in the brown sugar and granulated sugar, then stir in the egg, egg yolk and vanilla until well combined.
Separately, whisk together the flour, corn starch, oats, baking soda, salt and cinnamon. Add the dry ingredients and stir the dough together until smooth.
Cover the bowl with plastic wrap and refrigerate for 1 hour, to cool the butter back down and firm up the dough enough to scoop into balls.
Use a cookie scoop to portion the dough into 2-dozen balls. Use the tines of a fork to slightly flatten each ball.Place the dough balls back in an airtight container and freeze for 10 minutes while you preheat the oven.
Preheat the oven to 375 F, and line a large baking sheet with parchment paper.
Place the chilled dough balls 3 inches apart on the baking sheet and bake on the center oven rack for 8 minutes.
Remove from the oven. If desired, lightly press a square of chocolate into the center of each cookie. Cool the cookies for 2 minutes on the baking sheet, then transfer them to a cooling rack to cool completely.Sprinkle the cookies with the flaky finishing salt.
Once the cookies are cooled, the chocolate will still be quite melty. Go ahead and store the cookies in an airtight container, in a single layer, for several hours until the chocolate hardens again.