In a large microwave-safe bowl, melt the butter, about 1 minute. Allow the butter to cool momentarily so you don't scramble the egg. Add the egg, sugars, vanilla, and stir vigorously to combine, for about 1 minute. Stir in the peanut butter. Add the flour, baking soda, optional salt and stir until just incorporated. Stir in chocolate. Turn batter out into prepared pan, smoothing it lightly with a spatula. Bake for 18 to 22 minutes, or until center is just set and golden (a toothpick test should come out clean if you hit dough but because of the abundance of chocolate, hitting chocolate is more likely). Bars will firm up more in pan as they cool.
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