Preheat the oven to 350º F (180º C). Line a cupcake pan with cupcake liners.
In a large bowl, combine the flour, baking powder, baking soda, cocoa, and salt. Reserve.
Stir together hot water and instant coffee power until the coffee is dissolved. Reserve.
Add butter and sugar to a large bowl or the bowl of a stand mixer. Beat until well combined, creamy, and pale, about 3 minutes. Add sour cream, oil, vanilla, vinegar, and coffee mixture; mix until well combined.
Turn the mixer to low and add the flour mixture in three batches, alternating with buttermilk, beginning and ending with the flour mixture; beat until just combined, about 30 – 45 seconds. Do not overmix.
Divide batter evenly among liners, about three-quarters full each.
Bake for 20 - 23 minutes, or until a toothpick inserted in the center comes out clean. Remove from oven. Cool pan on wire racks for 5 minutes, then remove cupcakes from pan, place back on the rack, and cool to room temperature before frosting, about 1 hour.
Spread or pipe chocolate buttercream evenly on cupcakes. Sprinkle with chocolate jimmies, if desired. Serve.