Preheat the oven to 350, and line an 8x8 inch square pan with parchment paper, so that the paper hangs over the edges slightly.
Melt the butter in a small saucepan over medium heat. Remove from the heat, and stir in the cookie butter until melted.
Scrape the melted butter and cookie butter into a bowl, and add the granulated sugar and dark brown sugar. Stir until combined, although the mixture will be grainy.
Stir in the whole egg, egg yolk and vanilla, until the mixture is smooth with a "ribbony" consistency"
Separately, combine the flour, corn starch and salt. Add the dry ingredients and stir just until combined.
Briefly fold in the chocolate chips, reserving 2 tablespoons to sprinkle over the top of the batter.
Press the dough into the prepared pan. Sprinkle the 2 tablespoons chocolate chips over the batter.
Bake on the center oven rack for about 22 minutes. The bars should appear golden brown and slightly puffed.
Cool completely (at least 4 hours) before removing from the pan by lifting up the parchment paper, and cutting into bars. Sprinkle with extra sea salt or flaky finishing salt, if you like.