Cupcakes:

Preheat oven to 350° F / 175° C. Line a 12-cup muffin tin with silicone cupcake liners or paper cupcake liners.

In a large mixing bowl, whisk together all purpose flour, granulated sugar, cocoa powder, espresso powder, baking soda, and fine sea salt.

In a separate bowl, whisk together boiling water, vegetable oil, white vinegar, and vanilla extract

Make a well in the dry ingredients. Add wet ingredients and use a wooden spoon or rubber spatula to stir until just combined, with no dry pockets remaining. Do not overmix. (Batter will be lumpy.)

With a spouted measuring cup, pour batter into prepared muffin tin, dividing it equally between the cups.

Bake in preheated oven for 22-25 minutes or until nicely risen and a toothpick or cake tester comes out mostly clean.

Transfer to a wire rack to cool completely. Meanwhile, prepare the buttercream.

Buttercream:

In a large mixing bowl or the bowl of a stand mixer, whip butter on high speed until smooth and creamy.

Add confectioner’s sugar and cocoa powder, followed by milk, vanilla extract, and fine sea salt. Mix on medium-high speed until smooth and fluffy, about 2 minutes. If necessary, add a splash of milk or a teaspoonful of confectioner’s sugar to adjust the consistency. Chocolate buttercream should be thick and creamy, but not too stiff to pipe.

With a rubber spatula, scoop chocolate buttercream into a piping bag fitted with a star or other decorative piping tip. Pipe cupcakes with swirls of buttercream. If you don’t have a piping bag, you can spread the frosting with a small stepped palette knife or a butter knife.

Serve immediately. Store leftover eggless chocolate cupcakes in an airtight container in the refrigerator for up to 3 days, or freeze unfrosted cupcakes in an airtight container for up to 3 months.