Step 4: Now, Add the Wet Stuff!
Add eggs, warm espresso, sour cream, oil, and vanilla, and mix until smooth. For this step, I used my kitchen aid mixer, but a hand held mixer or even a wooden spoon would get the job done. Combining the dry and the wet ingredients activates the production of gluten in baked goods, which gives the baked good its structure. Mixing too much, though, produces too much gluten, making your delicate cupcakes tough.Therefore, watch what you're doing and only mix until everything is combined.