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Preheat the oven to 350°F. Line the wells of a 12-cup muffin pan and 6-cup muffin pan with papers, and grease the inside of the papers.
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To make the cupcakes: Place the pistachio nuts in the bowl of a food processor with half the flour. Process for 1 minute, until the nuts are finely ground. Sift the mixture through a strainer, returning any larger chunks to the machine for further grinding with the remainder of the flour.
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Whisk the ground nut/flour mixture with the cardamom, baking powder, and salt. Set it aside.
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In a large mixing bowl, beat the sugar, butter, and lemon zest until light and fluffy, 2 to 3 minutes. With the machine running, add the eggs one at a time, mixing until each egg disappears into the batter before adding the next. Scrape down the sides and bottom of the bowl.
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Add the sour cream and vanilla, mixing on low for 2 minutes, until the batter is uniform. Add the flour mixture and mix on low until just incorporated. Scrape the bowl one more time to make sure everything is evenly mixed.
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Scoop the batter by the scant 1/4-cup into the prepared cupcake pan, filling each well 2/3 full.
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Bake the cupcakes for 20 to 22 minutes, until a tester inserted in the middle of one of the center cupcakes comes out clean. Remove them from the oven and place them on a rack to cool.
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To make the ganache: Place the chocolate in a medium-sized heatproof bowl. Set the bowl aside.
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In a saucepan set over medium heat, bring the cream to a simmer. Pour it over the chocolate. Let stand for 3 to 4 minutes, to give the chocolate time to melt. Stir until the mixture is smooth, then add the butter and whisk until completely smooth and shiny.
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Run a spatula around the sides and bottom of the bowl to make sure no chocolate has settled there, add the salt, and stir until thoroughly combined.
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To assemble the cupcakes: Using a small offset spatula (or butter knife), place a large dollop (about 2 tablespoons) of ganache on each cupcake and smooth over in a circular motion from the center to the outer edges. For decoration, top each cupcake with chopped pistachios.