To prepare cake, preheat oven to 350F. Grease three 5-inch cake pans and line bottoms with parchment paper. Set aside.
In a large bowl, sift together flour, baking powder, baking soda and salt.
In a standing mixer with a paddle attachment, beat the butter and sugar on medium speed until light and fluffy. Add eggs, one at a time, and continue beating until well combined.
With the mixer on low, add half of the flour mixture, then milk, and the remaining flour mixture. Mix until combined.
Divide batter evenly into 3 portions. Fold in vanilla, espresso powder and cocoa powder in each portion respectively, until well mixed.
Pour batter into the prepared cake pans. Bake for 15 to 18 minutes. Remove from oven and let cakes cool completely.
Run a pairing knife around the edge of the cake to loosen it from the pan. Peel off the parchment paper circles.
To prepare coffee syrup, place all the ingredients into a saucepan. Bring to the boil and stir until the sugar has dissolved. Remove from the heat and cool.
To prepare frosting, combine mascarpone and powder sugar in a standing mixer with the paddle attachment on low speed until smooth.
Divide frosting into 3 portions. Fold in vanilla, espresso powder and cocoa powder respectively. Mix until smooth.
To assemble the cake, use a pastry brush to brush the coffee syrup over the tops of the cakes and allow to soak in. Place the chocolate cake layer on the serving plate and spread a layer of chocolate frosting. Place the espresso cake layer on top and spread a layer of espresso frosting. Place the vanilla cake on top and spread a layer of vanilla frosting on it.
To create the ombre effect, starting at the bottom, carefully smear the side of the chocolate cake with chocolate frosting, followed with espresso frosting on the side of the espresso cake, and vanilla frosting on the side of the vanilla cake. Use a mental spatula to smooth frosting all over the cake, to blend the colors together and create rustic stripes effect.
Dust the cake top with cocoa powder and garnish with espresso or chocolate macarons, if desired.