Condensed milk and browned butter brownies

Heat the oven to 325F, 160C convection if available.

Line a 9-inch square pan with a parchment sling, snipping each corner diagonally so it sits flat.

Melt the butter in a small pot over medium heat, then boil out its water. When it fizzes, pops and foams, stir constantly so the milk solids do not stick to the bottom.

When the butter smells like caramel and is just turning brown, whisk in the cocoa powder until smooth and let it boil for a minute; take it off the heat before it burns.

Whisk in the sweetened condensed milk, then the powdered sugar; undissolved specks are the browned butter solids.

Add the vanilla, a pinch of salt if the butter was unsalted, and optionally a little molasses dripped from the bottle.

Whisk in one egg yolk; the mixture has cooled enough that it will not scramble.

Whisk in the flour until smooth, with nuts if wanted. For chocolate chips, let the batter cool a bit first and stir in chilled chips with a wooden spoon.

Scrape the batter into the pan without smoothing it; it levels in the oven.

Bake up to 40 minutes. Pull at about 30 minutes, when a skewer comes out a little muddy, for fudgy; keep baking for chewier. These came out at 35 minutes.

Cool; after an hour lift out with the paper.

Variation, thick and cakey: use the whole egg, a full cup of flour and a teaspoon of baking powder.