Preheat the oven to 350 degrees F (175 degrees C).
Pour 1/2 cup melted butter into a 9×13-inch baking pan.
Combine 1 cup sugar, milk, and flour in a bowl. Pour mixture into the baking pan with the melted butter.
Combine pecans, eggs, 2/3 cup sugar, molasses, corn syrup, 1/3 cup butter, and vanilla extract in a bowl and pour into the baking pan over the flour mixture.
Bake in the preheated oven until set, 35 to 40 minutes. Serve warm.
