Preheat oven to 350F and line muffin tins with cupcake liners.
Whisk cocoa and hot water together until smooth, set aside.
In a separate bowl, whisk together flour, baking powder, baking soda, and salt.
Melt butter and sugar in a small saucepan over medium heat. Stir to combine.
Transfer butter to the bowl of a mixer fitted with the paddle attachment. Beat until mixture has cooled, approx 4mins.
Add eggs one at a time, beating until fully incorporated after each addition. Scrape sides of bowl as needed.
Add vanilla and cocoa mixture, beat until combined.
Reduce speed to low and alternate adding flour mixture and sour cream in 2 additions. Beat until just combined.
Divide batter evenly into lined cups. I use a large ice cream scoop for this. Fill about 3/4 full.*
Bake for 15-20mins or until a cake tester comes out mostly clean. Rotate the trays after 10mins.
Transfer to wire rack to cool for 10mins, then remove cupcakes from tins and cool completely on wire racks.
Decorate with Baileys Buttercream.