Blondie recipe built for a 6" pan. For 8x8, double everything more or less (190g flour, and an egg and a yolk vs 2 eggs) and bake for the same time.
Easily convertible to an as-good brownie recipe - add 50g chocolate chips to the sugar before melting together with butter, and decrease flour to 50g.
Preheat oven to 350F. grease and/or line a 6" square baking pan.
Gently melt butter (do not allow to steam or reduce).
Pour over sugar (and chocolate chips, if making brownies) in a mixing bowl. Add vanilla. Stir til combined.
Ensure sugar mixture is not steaming; stir in beaten egg.
Add flour and baking powder and salt, and stir with a rubber spatula til just smooth.
Pour batter into pan. Optionally top (30g crushed pecans and/or 15g chocolate chips, or similar) and bake (middle rack) for 25-28 minutes, until toothpick comes out mostly clean; a few crumbs is fine. Allow to fully set on wire rack, 2 hours, before cutting. For fudgier brownies, store in fridge.