Preheat your oven to 350°F (175°C). Whisk together flour, salt, baking soda, and extra flour in a bowl. In another bowl, cream butter with both sugars until smooth and fluffy. Add vanilla and egg, then gradually mix in the dry ingredients. Stir in chocolate chips. Press the dough evenly into your tart pan, building up the sides slightly. Chill for 5 minutes, then bake for 22–25 minutes until golden. While still warm, press the center down gently with a metal cup to create space for the caramel layer.
Warm the heavy cream slightly and set aside. In a saucepan, combine sugar, corn syrup, and water, and stir until the sugar dissolves. Let it boil without stirring until it turns a deep golden color. Remove from heat and carefully whisk in the warm cream (it will bubble up). Stir in butter, sea salt, and vanilla until smooth. Let the caramel cool for about 10 minutes, then pour it into the baked crust. Refrigerate while preparing the glaze.
Melt butter and chocolate chips together in a microwave-safe bowl, stirring every 20 seconds until smooth. Add corn syrup and mix well. Allow it to cool slightly so it thickens, then pour over the caramel layer. Spread evenly using an offset spatula, then sprinkle lightly with fleur de sel or flaky sea salt. Chill until set before slicing.