1) Over a double-boiler, add the butter and brown sugar and whisk it together until the butter is melted and sugar is dissolved.
2) Let this cool for a minute until it’s warm to the touch then whisk in the eggs, vanilla, salt, and flour last. Try not to over-mix the batter.
3) Stir in the mix-ins.
4) Pour batter into a lightly buttered 9×13 baking dish and bake at 350 degrees for 30-40 minutes depending on how firm you like your blondies.
5) Let them cool for 20-30 minutes on a wire cooling rack before digging in.