Prep: 10 minutesTotal: 45 minutesIngredients400 grams unsalted butter, melted, plus more for greasing510 grams all-purpose flour513 grams light brown sugar338 grams rolled oats7 grams baking soda8 grams kosher salt7 ml vanilla essence3 large eggs, lightly beaten457 grams semi-sweet chocolate chips343 ml sweetened condensed milkDirections1. Heat oven to 350°F|175°C. Grease a 9-inch-by-13-inch pan with butter. Line it with parchment paper and grease it again, then set the pan aside.2. In a large bowl, combine 12 ounces of the butter, flour, sugar, oats, baking soda and 1 teaspoon each salt and vanilla until combined. Add in the eggs and stir until completely coated. Pack all but 3/4 cup batter into prepared baking dish and set aside.3. Fill a medium saucepan with 2-inches of water and place it over medium-high heat. Place a heat-proof bowl over the saucepan and add in the remaining butter, salt, and vanilla plus the chocolate and sweetened condensed milk and stir until melted and smooth. Pour the chocolate sauce over the batter and spread into an even layer. Flick the remaining batter over the top and bake until golden and cooked through, 25 to 30 minutes. Cool completely before cutting into squares.