Preheat the oven to 350 F. Line a 9-by-13-inch metal baking pan with parchment paper.

In a large mixing bowl, whisk the flour with the baking soda and salt. Add the oats and brown sugar and mix until combined. Add the butter and mix on the lowest speed (or by hand) until the butter is uniformly mixed in–it will remind you of crisp topping, quite buttery and clumpy.

Pat half of the oat mixture into the prepared baking pan. Bake for 15 minutes.

Scatter half of the chopped chocolate and half of the hazelnuts over the crust. Drizzle half of the dulce de leche mixture on top of these. Crumble half of the remaining oat mixture evenly into the pan. Pat it gently, so that it is level but not smashed in. Cover this second layer with the remaining chocolate, oats and dulce de leche, in the same manner as the first layer, ending with the remainng oat mixture. Sprinkle with the coarse sea salt before lightly patting.

Bake for about 35 minutes, or until the edges are set. Let stand uncovered at room temperature overnight. After slicing the next day, store tightly sealed.