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.
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.
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.
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.