Bring sugar, water, and instant espresso powder to a boil in a small pot over med-high heat. Simmer for 1min. Remove from heat and stir in coffee liqueur or rum if desired. Set aside to cool.
Preheat oven to 350F. Grease and flour two 8" cake rounds and line with parchment.
In a medium bowl, whisk flour, baking powder, and salt until well combined. 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. Add vanilla.
Alternate adding flour mixture and buttermilk, beginning and ending with flour (3 additions of flour and 2 of milk). Fully incorporating after each addition.
Bake for about 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. Cool completely.
Beat butter and mascarpone until fluffy (2 mins). Add powdered sugar one cup at a time. Add vanilla and beat until fluffy (3 mins). Do not overmix.
Cut each cake layer in half horizontally.
Place one layer of cake on a cake stand or serving plate. Brush generously with about 4 Tbsp coffee syrup. Dust with cocoa powder if desired (I did).
Top with approximately 3/4 cup of mascarpone buttercream and spread evenly.
Repeat with remaining layers and crumb coat the cake. Chill for 20mins.
Frost the cake with the remainder of the buttercream and smooth the sides and top.
Dust top with cocoa powder and/or sprinkle with chocolate shavings, and press lady fingers into the sides if desired.