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Preheat oven to 350 degrees.
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In a large bowl, whisk together the flour, baking soda, baking powder, and salt. Set aside.
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Put butter and sugar in a large saucepan and stir until melted and combined. Pour into the mixing bowl of an electric mixer and let cool for 5 minutes.
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In the meantime, boil water in a small saucepan. Remove from heat and whisk in cocoa powder until smooth. Set aside.
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Add in the eggs one at a time. Add in the vanilla and cocoa mixture. With speed on low, alternate adding the flour mixture and the sour cream, scraping the sides of the bowl frequently to ensure it’s well mixed.
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Fill cupcake liners 2/3 of the way full. Gently press 1 mini Reese’s Cup into the center of each cupcake.
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Bake for 18-20 minutes until a toothpick inserted into the center comes out clean. Set aside to let cool.
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Once the cupcakes are cool, beat peanut butter and butter together until creamy. Add in 2 cups of the powdered sugar. Add in vanilla and salt. Add remaining powdered sugar and heavy cream as necessary until creamy and smooth.
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Pipe the frosting onto the cupcakes – I used Wilton Tip 1A for this style! If you want, melt the chocolate chips with ~2 tbsp. of veggie oil in 30-second intervals. Drizzle the chocolate onto the top of each cupcake and then press 1/2 of a Reese’s Cup into the top of each cupcake. Dig it open and enjoy!