Heat oven to 350°F. Butter an 8×8-inch baking pan, or line the bottom and two sides of one with parchment paper, buttering the exposed parts.
Melt butter in a small saucepan; I like to keep cooking it, stirring occasionally, until it becomes toasty and browned at the bottom and has an unbelievable aroma. It’s definitely worth the trouble here. Pour into a medium or large bowl and whisk in sugar until smooth. At this point, it should be cooled to lukewarm, but if it’s not, let it rest for a few minutes until it is, then whisk in egg, vanilla, and sea salt. Stir in flour and additions of your choice.
Scrape into prepared pan and smooth with spatula; the batter is thick and will take some nudging. Bake for 20 to 25 minutes, until lightly browned at edges but looks a tiny bit underbaked. (I always err on the side of caution with baking times — nobody ever complained about a gooey-middled cookie.) Let blondies cool in pan, or just let them cool in it for 10 minutes and use the parchment sling to slide them onto a cutting board. Cut into 16 or more squares.