In a heatproof bowl set over simmering water (double boiler), melt 2 ounces of unsweetened baking chocolate, 3 ounces of semi-sweet baking chocolate, and 9 tablespoons of butter together. Stir frequently until smooth and glossy. Remove from heat and let it cool slightly.
In a large bowl, whisk together the granulated sugar and eggs until well combined and slightly thickened. Stir in the vanilla extract.
Slowly pour the cooled chocolate and butter mixture into the egg mixture, folding gently to combine. Add the cocoa powder and mix well.
Sift the all-purpose flour and salt together, then fold into the wet ingredients until just combined. Avoid overmixing to keep brownies tender.
Fold in the chopped pecans and 1 cup of the caramel bits evenly throughout the batter.
Grease the 9x13-inch pan with the remaining 4 tablespoons of butter, then spread half of the batter evenly in the pan. Sprinkle half of the remaining semi-sweet chocolate chunks and ½ cup of caramel bits over the batter.
Spread the remaining batter over the chocolate and caramel layer. Top with the leftover semi-sweet chocolate chunks and pecan halves, gently pressing them into the batter.
Bake in a preheated 350°F (175°C) oven for 30-35 minutes or until a toothpick inserted near the center comes out with a few moist crumbs. While still warm, mix the 2 teaspoons milk with the remaining caramel bits and drizzle over the top for extra gooeyness.
Allow the brownies to cool completely in the pan on a cooling rack before slicing into squares. This helps the caramel set and makes cutting easier.