Preheat oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners.

In a small bowl, combine boiling water and instant espresso powder. Set aside to cool slightly.

In another bowl, whisk together the flour, baking powder, baking soda, and salt.

In a large mixing bowl, beat the butter, oil, and sugar until light and creamy, about 2–3 minutes.

Add the eggs one at a time, beating well after each addition.

Gradually add the dry ingredients to the bowl, mixing until just combined.

Pour in the cooled coffee mixture, sour cream, and vanilla and mix until smooth.

Divide the batter evenly among the cupcake liners, filling each about ¾ full.

Bake for 15–20 minutes, or until a toothpick inserted into the center comes out clean.

Let the cupcakes cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.

Beat the butter with 2 cups of powdered sugar in a large bowl until light and smooth.

Add the remaining 1 cup of powdered sugar and mix until fully incorporated.

Gradually add the brewed coffee, one tablespoon at a time, beating until the frosting is creamy and fluffy.

Continue to beat until smooth and spreadable.

Transfer the buttercream to a piping bag fitted with a Wilton 1 tip and frost the cooled cupcakes.