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Basically perfect and dead simple. From here, halved 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.
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 in a mixing bowl and whisk til smooth.
Add room-temperature egg (room temperature so it isn't so much colder than the butter and sugar that it scrambles when it hits it, but that isn't a problem if you don't overheat the butter when you melt it.)
Add flour and baking powder and salt and stir with a rubber spatula til smooth, approx 50 strokes. Don't overdo it.
Mix in additional ingredients, spatula into pan, and bake (middle rack) for 25-30 minutes, until toothpick comes out mostly clean; a few crumbs is fine. Cool at least a little before cutting.
Current best advice for mix-ins: 20g (a scant tablespoon) of Fluff gently mixed into the batter before pouring into the pan - do not over-stir, batter should be zebra-striped - topped evenly and densely with enough crushed pecans that you can no longer see the batter (50-60g. Don't skimp.) and then dotted with good quality semi-sweet chocolate chips (10g). The nuts will express oils as the blondies bake, darkening the surface - do not mistake this for over-baking, check with a toothpick. No chunky mix-ins means even baking and allows you to precisely target the texture you want, but it may bake slower than expected due to the nut layer.