Preheat oven to 350° and line 2 muffin tins with liners. In a large bowl, whisk together flour, espresso, salt, baking powder, and baking soda.
In another large bowl using a hand mixer, beat butter and sugar together. Add sour cream and beat until incorporated. Scrape down the sides of the bowl, then add eggs one at a time, then add vanilla. Add dry ingredients to wet and beat until just combined. Add coffee and beat until just combined.
Using a large cookie scoop, fill liners ¾ full with batter. Bake until a toothpick inserted in middle comes out clean, 20 minutes. Let cool completely.
Make the frosting: In a large bowl using a hand mixer (or in the bowl of a stand mixer with the whisk attachment), beat butter until light and fluffy and pale in color, about 3 minutes. With the mixer running on medium speed, slowly add cold condensed milk. Add vanilla and salt and beat until frosting is stiff.
Transfer frosting to a piping bag and pipe onto cooled cupcakes.
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