Preheat oven to 325° and generously butter (2) 9 inch round cake pans and then lightly dust with flour. Cut parchment paper to fit and place in bottom of pan. Set aside.
Using a medium bowl, whisk flour, cocoa powder, baking soda and salt together, set aside.
In a large bowl, beat butter with an electric mixture until creamy then add brown sugar and beat for approximately 2 minutes until mixture is blended and smooth.
Add eggs one at a time beating well between additions then add vanilla.
Add flour mixture, alternating with buttermilk in 2 additions, and beating just until blended after each addition.
Heat water in microwave for 60 seconds, add instant espresso, stir until dissolved.
Gradually add hot espresso mixture into cake batter beating until smooth.
Pour batter equally into both cake pans and place on center rack in preheated 325° oven.
Bake until toothpick inserted comes out clean, about 40 minutes.
Remove from oven and cool on rack for 15 minutes then run a knife along the edge to loosen up the cake.
Invert cake on rack, remove parchment paper then invert cakes upright to cool completely.
In a large bowl, whisk cocoa powder and espresso together.
Using a small saucepan, bring 1 cup of whipping cream to a boil.
Slowly pour cream over the cocoa/espresso mixture whisking until dissolved completely.
Add remaining whipping cream and sugar to cocoa mixture, stir until dissolved.
Place in fridge for approximately 2 hours until chilled.
Once chilled, add mascarpone to cocoa/espresso mixture.
Beat on low speed until smooth then increase to medium high speed and beat until mixture is thick and medium-firm peaks form when beaters are lifted, about 2 minutes (DO NOT over beat otherwise the mixture will curdle).
Before adding frosting, use a pastry brush and brush off any crumbs on the top and sides of each cake layer.
Place first cake layer, top side up, and add a thin layer of frosting to start. Add more frosting and once desired thickness is obtained, add the second cake layer, top side up again.
Continue to frost cake and top with chocolate curls, optional.
Recipe from bonappetit.com