Bring sugar, water, and espresso powder to a boil in a small pot over med-high heat. Simmer for 1min. Remove from heat and stir in Kahlua. Set aside to cool.
Cake:
Preheat oven to 350F. Grease and flour three 6" cake rounds and line with parchment.
In a medium bowl, whisk flour, baking powder, and salt until well combined. Set aside. Combine Kahlua and buttermilk. Set aside.
Using a stand mixer fitted with a paddle attachment, cream butter and sugar on med-high until pale and fluffy (approx 3mins). Reduce speed and add eggs one at a time fully incorporating after each addition.
Alternate adding flour mixture and buttermilk mixture, beginning and ending with flour (3 additions of flour and 2 of milk). Fully incorporating after each addition.
Bake for 30-35mins or until a toothpick inserted into the center comes out mostly clean.
Place cakes on wire rack to cool for 10mins then turn out onto wire rack. Poke holes into the cakes using a bamboo skewer and brush generously with Kahlua syrup.
Allow cakes to cool completely.
Mocha Buttercream:
Place egg whites and sugar into the bowl of a stand mixer, whisk until combined.*
Place bowl over a pot with 1-2" of simmering water and stir constantly with a whisk until the mixture is hot and no longer grainy to the touch or reads 160F on a candy thermometer (approx. 3mins).
Place bowl on your stand mixer and whip on med-high until the meringue is stiff and cooled (the bowl is no longer warm to the touch (approx. 5-10mins)).
Switch to paddle attachment. Slowly add cubed butter and mix until smooth.**
Add cooled melted chocolate and espresso powder. Mix until combined. Slowly stream in Kahlua and whip until smooth.
Assembly:
Place one layer of cake on a cake stand or serving plate. Top with approximately 2/3 cup of buttercream. Repeat with remaining layers and crumb coat the cake. Chill for 20mins.
Frost the top of the cake with approximately 2/3 cup of buttercream. Do a decorative swirl using a large offset spatula. Smooth any frosting that overhangs the sides.
Using a large petal tip, pipe ruffles around the cake in horizontal rows starting at the top.
* Ensure there is NO trace of egg yolks in your whites and that your mixer bowl and whisk is completely grease free or your meringue won't stiffen.
** The buttercream may look like it's curdled at some point. Keep mixing until it is completely smooth.