Preheat the oven to 350°F. Grease and line a metal 9×13 pan with parchment paper. In a stand mixer, combine the butter and granulated sugar. Mix until combined. Add vanilla, egg, and egg yolk, and mix until incorporated.

With the mixer on low, add flour, salt, baking soda, cream of tartar, and corn starch. Mix until just combined. Scoop the batter into the prepared pan and press it evenly with a spatula or lightly wet hands. Poke dough several times with a fork. Bake for 20-22 minutes, or until the edges are golden brown. Remove from the oven and let cool completely before assembling.

Add sugar to a saute pan over medium-high heat. Stir occasionally with a metal whisk. The sugar will clump and melt, turning a deep amber color. Stir continuously until all the sugar is dissolved. Once melted, stir in butter one tablespoon at a time. After the butter melts completely, slowly drizzle in warm heavy cream until combined. The mixture will bubble and steam, so be careful! Remove from heat and stir in kosher salt.

Break up the chocolate and place it in a medium-sized bowl. Heat the cream in the microwave for 30-45 seconds or until it bubbles. Pour over the chocolate and let sit for 3 minutes before whisking. If the chocolate doesn’t completely melt, microwave for 20 additional seconds and whisk until smooth. Let cool for 10 minutes.

Once the cookie bars are completely cooled, pour the caramel on top and spread evenly. Refrigerate for 15 minutes.

Pour ganache over the caramel and spread to coat. Sprinkle with flaky sea salt. Refrigerate for 30-60 minutes to make slicing easier. Store in the refrigerator for up to 5 days.