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Basically perfect and dead simple. From here, halved for a 6" pan. For 8x8, double everything more or less (191g 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. Whisk butter and light brown sugar in a mixing bowl 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 really isn't a problem if you don't overheat the butter when you melt it, or if you cool the butter and sugar a bit before you add the egg.)
Add flour and baking powder and salt and stir to combine. Mix in additional ingredients, spatula into pan, and bake (middle rack) for 25ish minutes. Cool at least a little before cutting.