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.
In a small measuring cup, combine room temperature milk & espresso powder. Stir until dissolved. 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 vanilla bean paste.
Alternate adding flour mixture and espresso milk, 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 to cool completely.
Vanilla Latte Buttercream:
Place egg whites and sugar into the bowl of a stand mixer, whisk until combined.**
Place bowl over a double boiler on the stove and whisk constantly until the mixture is hot and no longer grainy to the touch (approx. 3mins). Or registers 160F on a candy thermometer.
Place bowl on your stand mixer and whisk 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 vanilla bean paste and espresso powder. 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 and sides of the cake and smooth with an icing smoother.
Do a swirl pattern on the sides and top of the cake. Rotate the turntable while pressing a small offset spatula into the frosting. Start at the bottom and slowly move up to the top of the cake.
Pipe small dollops on top using a 6B tip with remainder of frosting. Decorate with whole coffee beans if desired.
* If you can't find very fine espresso powder, you can dissolve the 2Tbsp in 1 Tbsp hot water. Cool completely before adding to the frosting. ** 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.