Preheat oven to 350°F and line a cupcake pan with liners.
In a large bowl, add the chocolate cake mix, pudding mix, milk, oil, eggs plus the extra yolk, sour cream, vanilla, and espresso powder. Mix until smooth and slightly thick, just until everything is fully combined.
Fill each cupcake liner about 2/3 full. Bake for 18 to 22 minutes, until a toothpick inserted in the center comes out with a few soft crumbs. Let the cupcakes cool completely before frosting.
To make the chocolate buttercream, beat the softened butter in a large bowl until smooth and creamy. Add the powdered sugar and cocoa powder, mixing until combined. Pour in the heavy cream and vanilla, then beat until light, fluffy, and easy to spread or pipe. Add more cream if needed to reach your desired consistency.
Pipe or spread the frosting onto the cooled cupcakes. Finish with white sprinkles, a light dusting of sanding sugar, and a happy anniversary topper.