Preheat the oven to 350°F. Line an 8-inch-square baking pan with enough foil to extend 2-inches over the sides; spray the foil with nonstick spray.
In a large bowl, use an electric mixer to combine the butter, brown sugar and vanilla until well mixed. Add the flour, beating until it is blended and the mixture is crumbly. Press the crumb mixture into the bottom of the pan. Bake until the crust is lightly browned at the edges, about 15 minutes.
PREPARE THE TOPPING:
Combine the caramels and milk in a microwaveable bowl. Microwave for 2 minutes until the mixture is hot, stirring once halfway through the cooking time. Stir again until the mixture is smooth. Pour the topping onto the hot baked crust and spread evenly, leaving a 1/4-inch border along the edge. Place the pecans on top.
Return the pan to the oven and bake just until the topping is bubbling in the center, about 12 minutes longer. Melt the two chocolates separately in the microwave and scoop into individual plastic zip baggies. Snip a tiny corner of each zip bag and drizzle in a zigzag manner on top of the caramel topping. Cool completely on a rack.
Using the foil overhang as handles, transfer to a cutting board. Gently peel away the foil and cut into 16 bars.
Notes
White chocolate is sometimes tricky to melt. If yours isn't melted well, just add about 1/2 teaspoon shortening to the white chocolate and it should smooth it out and melt a little easier.