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Combine the vegetable oil, espresso, brown sugar, cocoa powder and chocolate in a large microwave-safe bowl and microwave until hot, about 2 minutes. Whisk until smooth, then set aside to cool, about 15 minutes.
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Whisk the flour, granulated sugar, baking soda and fine salt in a medium bowl. Add the buttermilk, eggs and vanilla to the cooled chocolate mixture and whisk until smooth. Add the flour mixture and continue whisking until combined.
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Line a 12-cup muffin pan with paper liners. Divide the batter among the cups, then sprinkle with the chopped bacon. Bake until the cupcakes spring back when lightly pressed, 20 to 25 minutes. Let cool 10 minutes in the pan, then remove to a rack to cool completely.