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Heat the oven to 350 degrees F. Line muffin pans with paper liners.
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Cream the butter and sugar in the bowl of an electric mixer fitted with the paddle attachment on high speed, until light and fluffy. On medium speed, add the eggs, 1 at a time, then add the yogurt and vanilla. Scrape down the sides and stir until smooth.
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Sift together the flour, cornstarch, salt, and baking soda in a bowl. With the mixer on low speed, add the flour mixture to the butter mixture until just combined. Gently stir in peanuts. Fill the cupcake liners to the top with batter. Bake in the center of the oven for 20 to 30 minutes, until a toothpick comes out clean. Cool to room temperature.
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For nougat layer, use a spatula to fold marshmallow fluff and peanut butter together until combined. Spread a thin layer on cooled cupcakes.
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For frosting, melt the 2 kinds of chocolate in microwave, stirring as needed. Set aside to cool slightly.
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Beat butter until creamy; add 1 ½ cups powdered sugar ½ cup at a time until fully incorporated. Add chocolate, heavy cream, vanilla, and salt. Beat until everything comes together and add powdered sugar as needed. Spread on top of nougat layer. Top with salted peanuts.