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Set oven to 350 degrees (adjust oven rack to middle position).
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Line an 8-inch square metal baking pan with parchment paper or foil, then spray lightly with cooking spray.
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In a small bowl whisk together flour and baking powder; set aside.
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In another bowl whisk cocoa powder, water, vanilla and espresso powder; set aside.
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In a medium microwave-safe bowl microwave the butter and chopped chocolate together on 50% power until melted (about 1 minute); whisk until the mixture is smooth, then add in sugar and salt and whisk until completely combined.
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Add in the cocoa mixture and then the egg; whisk until combined.
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Stir in the flour mixture until JUST incorporated (do not overmix!).
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Pour the batter into prepared baking pan and smooth out on top.
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Bake for about 20-25 minutes or until a toothpick inserted in the middle comes out with a few moist crumbs attached (rotate the pan about halfway through baking, do not overbake the brownies).