Preheat the oven to 400 degrees F. Line 2 standard cupcake pans with 18 liners. Set aside.
In a mixer, cream butter until smooth. Add the sugars, eggs, and vanilla and mix until fluffy, about 3 minutes.
Add the cocoa, espresso powder, baking soda, powder, and salt; mix well. Add one cup of flour, then half the sour cream and mix. Repeat with the remaining flour and sour cream.
Slowly add the hot coffee and mix until smooth. The batter will be thin.
Fill cupcake liners ⅔ of the way. Place tins in the oven and drop the temperature to 350 degrees. Bake 15-16 minutes, checking at 15 for proper doming.
Remove from oven and allow to cool in pan for 10 minutes, then move to cooling rack. When cool, use a cupcake corer or small knife to make a well in the center of each cupcake.
Espresso Fudge Sauce
Bring cream, corn syrup, brown sugar, espresso powder, cocoa, salt, and half of chopped chocolate to a boil in a small heavy saucepan over medium heat, stirring until chocolate is melted.
Reduce heat and cook at a slow boil, stirring occasionally, for about 5 minutes.
Remove from heat. Stir in vanilla, butter, and remaining chocolate until smooth. Cool to warm. Fill cored cupcakes with fudge sauce.
Hazelnut Espresso Brittle
Cover a large baking sheet with parchment paper. Sprinkle nuts & coffee beans evenly down the middle of the sheet.
Place sugar in a small, heavy saucepan over medium heat, and cook until sugar dissolves, stirring gently as needed to dissolve sugar evenly (about 2 minutes). Continue cooking 1 minute or until golden (do not stir).
Drizzle thin ribbons of caramelized sugar over nuts. Let stand 10 minutes or until firm; break into small pieces.
Mascarpone Buttercream
Beat powdered sugar and butter in a large bowl with an electric mixer on medium-high speed until thick and creamy, 4 to 5 minutes. Beat in vanilla.
Add mascarpone cheese, and beat on low speed just until combined, stopping to scrape down sides of bowl as needed. (Do not overmix!)
Top filled cupcakes with buttercream. Drizzle thin lines with fudge & top with brittle pieces.