In a large mixing bowl or bowl of a stand mixer, combine the butter, brown sugar, sugar salt, and 2 teaspoons vanilla. Using a stand mixer or hand held mixer beat the mixture until it is smooth, scraping down the sides a few times. Add the flour and water, beat until combined.

Stir in the chopped pecans.

Place the dough on a large piece of parchment paper or plastic wrap. Using your hands, shape it into a log 10 inches long and about 2 inches in diameter. Wrap the dough up and place it in the freezer for 30 minutes or the fridge for 2-3 hours.

While the dough chills, caramelize the sweetened condensed milk. Pour the sweetened condensed milk into a non-sick sauce pot. Simmer the sweetened condensed milk over low heat for 20-30 minutes, stirring often to ensure the sauce is not burning. Once the sauce turns a very light shade of caramel stir in 1 teaspoon vanilla and remove from the heat.*

Preheat oven to 350 degrees. Line a baking sheet with parchment paper.

Unwrap the dough and place it on a cutting board. Using a knife, cut the log into 1/4 inch slices. Arrange the slices 1 inch apart on baking sheet. Bake the cookies for 10-14 minutes, their edges will be golden brown. Allow the cookies to cool before filling.

Once the cookies are cool, spread the bottom half of the cookies with the caramelized sweetened condensed milk. The sweetened condensed milk will be very thick, just be gentle spreading it onto the cookies. If needed, warm the sweetened condensed milk over low heat until just spreadable. Add the top cookie to make a sandwich and place the sandwich cookies on a parchment lined baking sheet.

Melt the chocolate over a double broiler or in the microwave on full power, stirring every 30 seconds until smooth. Drizzle the chocolate over each cookie and if desired sprinkle with flaky sea salt. To harden the chocolate, place the tray of cookies in the fridge for 5 minutes. Store cookies in an airtight container for up 4 days at room temperature.