1. Preheat the Oven: Preheat your oven to 160ºC/320ºF (conventional). Line a 12-cup cupcake tray with cupcake liners.
2. Boil the Water: Start by boiling 120 g of water. Weigh it accurately and set it aside to cool slightly.
3. Mix Dry Ingredients: In a large mixing bowl, combine all the dry ingredients – granulated sugar, Dutch-processed cocoa powder, all-purpose flour, instant espresso powder, baking powder, baking soda, and salt. Stir well to ensure even distribution.
4. Mix Wet Ingredients: In a separate bowl, whisk together the wet ingredients – sour cream, vegetable oil, eggs, vanilla extract, and the slightly cooled boiled water. Mix gently to combine; avoid over-mixing to prevent tough cupcakes.
5. Combine Wet and Dry Ingredients: Slowly pour the wet mixture into the dry ingredients, gently folding them together. Stir until there are no lumps, but be careful not to overmix.
6. Fill the Cupcake Liners: Divide the batter evenly among the 12 cupcake liners in your prepared tray.
7. Bake: Bake the cupcakes in the preheated oven for 20-23 minutes or until a toothpick inserted into the center comes out clean. Let the cupcakes cool in the pan for about 5 minutes, then transfer them to a wire rack to cool completely.

Making the Creamy Peanut Butter Frosting

1. Mix Butter and Peanut Butter: In a stand mixer fitted with the paddle attachment (or using a hand mixer), beat the butter and peanut butter together on medium-high speed for about 2 minutes until light and fluffy.
2. Add Powdered Sugar, Milk, and Vanilla: Sift the powdered sugar to avoid lumps, and then gradually add it to the butter mixture along with the milk and vanilla extract. Mix on low speed until fully incorporated, then increase to medium speed and beat for another minute until the frosting is smooth and creamy.