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Cupcakes:
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Preheat to 350 F. Line a 12-cup muffin tin with paper liners.
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Place butter, chocolate and cocoa powder in a medium heatproof bowl. Microwave in 30 second spurts on 50% power, stirring in between, until the butter and chocolate are melted and the mixture is smooth. Set the mixture aside to cool until just warm to the touch.
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In a small bowl, whisk together the flour, baking soda, and baking powder. Whisk the eggs in a medium bowl to combine, then add the sugar, vanilla and salt and continue whisking until fully incorporated. Add the cooled chocolate mixture to the bowl and whisk to combine. Sift one-third of the flour mixture into the bowl and whisk gently to combine. Add the yogurt or sour cream, whisk to combine and finally, sift the the rest of the flour mixture into the bowl and whisk until everything is well combined - the batter will be thick.
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Divide the batter evenly among the muffin tins, filling each about 2/3 full. Bake 18-20 minutes. Cool the cupcakes in the pan on a cooling rack for about 15 minutes and then remove each cupcake from the pan and place directly on the rack to cool completely before frosting.
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Buttercream:
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In a stand mixer fitted with the whisk attachment, beat the butter and peanut butter at medium-high speed until smooth, about 20 seconds. Add the confectioners' sugar and salt and beat at medium-low speed until most of the sugar is moistened, about 45 seconds. Scrape down the sides of the bowl and beat at medium speed until mixture is fully combined, about 15 seconds. Scrape the bowl again, then add vanilla and heavy cream, and beat at medium speed until incorporated, about 10 seconds. Finally, increase speed to medium-high and beat until the buttercream is light and fluffy, about 4 minutes, stopping to scrape down the bowl once or twice.
