Preheat oven to 350°F (175°C). Line an 8×8-inch baking pan with parchment paper, leaving an overhang on two sides, and set the pan aside.
In a large bowl, combine ½ cup melted salted butter and 1 cup packed light brown sugar. Stir until the sugar is moistened and the mixture is smooth.
Add 1 large egg and ½ tablespoon vanilla extract to the bowl. Stir vigorously until fully incorporated and the mixture is smooth.
Add 1 cup all-purpose flour and stir or beat until no dry streaks of flour remain and the batter is smooth; the batter should be thick and may begin to pull away from the sides of the bowl.
Fold in 1 cup white chocolate chips and ½ cup dried cranberries until evenly distributed.
Spoon the batter into the prepared pan and spread it evenly with a spatula.
Bake for 20–25 minutes, until the top is set and a toothpick inserted into the center comes out mostly clean with a few moist crumbs (not wet batter).
Remove the pan from the oven and, if desired, sprinkle with sea salt. Allow the blondies to cool completely in the pan on a wire rack (or countertop) before using the parchment overhang to lift them out and slice into squares.