Preheat oven to 350° F. Line two standard muffin tins with paper liners and set aside.
In a medium bowl, whisk together flour, baking soda and salt. Set aside.
In a separate medium bowl, whisk together cocoa powder and espresso powder. Add the boiling water and whisk until smooth. Allow mixture to sit until room temperature, then whisk in sour cream and vanilla. Set aside.
In a the bowl of a stand mixer (or using a hand mixer), combine butter and sugar. Beat on medium-high speed until light and fluffy, about 3 minutes. Add eggs one at a time beating until combined after each addition. Scrape down the sides of the bowl as necessary.
Add one-third of the flour mixture to the large mixing bowl and mix on low speed just until combined. Add half the chocolate mixture and mix until combined. Repeat process, then end with remaining third flour mixture. Use a silicone spatula to scrape the bottoms and sides of the bowl. Mix once more for 10 seconds to ensure all ingredients are incorporated.
Transfer batter to prepared muffin tins and fill each cup two-thirds full (I like to use an ice cream scoop for this step). Bake cupcakes 18 to 22 minutes or until a tooth pick inserted in the center comes out with little to no crumbs. Allow cupcakes to cool for 5 minutes in the pan before transferring them to a wire rack to cool completely.
In the bowl of a stand mixer, whip the butter on medium-high speed until fluffy, about 5 minutes.
With the mixer on low speed, gradually add in the confectioners' sugar until completely incorporated. With the mixer on medium speed, add the vanilla and salt, and beat for a minute, until fluffy. Add milk until desired consistency is reached.
Decoration
For ghost cupcakes: use a large round frosting tip to pipe a cone shape on top of a cupcakes. Decorate with mini chocolate chips for the eyes.
Melt the chocolate in the microwave on 50% power level, stopping to stir every 20 seconds until smooth. Transfer to a zip-top bag and set aside to cool slightly.
For spider web cupcakes: Use an off-set spatula to spread frosting in an even layer over the cupcake. Snip a small hole in the corner of the baggie with melted chocolate. Place a dot in the center of the cupcake, then three circles around the dot (like a target). Use a toothpick to pull the chocolate from the center out to the edge. Repeat about 5 to 7 times to create a spider web.
For skull cupcakes: Use an off-set spatula to spread frosting in an even layer over the cupcake. Snip a small hole in the corner of the baggie with melted chocolate. Pipe two large eyes and two small dots for the nose. Pipe a large “stitched” smile for the mouth (like Jack Skellington).
Notes
Cupcakes: If you prefer, you can use store-bought cupcake mix and follow the instructions on the box.Frosting: If the frosting is too soft, put it in the refrigerator for 15 to 20 minutes, then mix for 5 seconds. You can also use store-bought frosting if you prefer.Decoration: I suggest reading my tips for melting chocolate if you need guidance.Store cupcakes in an airtight container at room temperature or in the refrigerator for up to 3 days. Freeze cupcakes for up to 1 month. Defrost in the refrigerator overnight.