Preheat oven to 350F. Butter four 9" cake pans and line the bottoms with parchment paper. Set aside.

Separate the egg whites and egg yolks.

In the bowl of a stand mixer fitted with the whisk attachment, beat the egg whites, sugar, and vanilla until stiff peaks, about 6-8 minutes.

While whisking on Medium speed, gradually add the egg yolks.

In a separate bowl, whisk the flour, baking powder, and salt together. Sift into the egg whites mixture and mix just until combined.

Divide the batter between the prepared cake pans.

You may need to bake in batches of two cake pans at a time. Bake for 15-18 minutes, until set in the center and golden on top.

Let cool in the cake pans for 10 minutes, then flip upside down on a clean, non stick surface to cool completely.

Mix all ingredients until well combined.

In the bowl of a stand mixer fitted with the whisk attachment, combine the egg whites, cream of tartar, and 1/3 cup sugar. Beat on HIGH speed until stiff peaks, about 6-8 minutes.

In the meantime, combine the water and remaining 1 1/2 cups sugar in a saucepan over medium heat. Use a candy thermometer to heat up to 235F, stirring occasionally.

While the stand mixer is on Medium speed, gradually pout in the sugar syrup. Beat until cool to touch, about 8-10 minutes.

If the meringue is still warm, place in the fridge for 10-30 minutes to cool down.

Switch out the whisk attachment for the paddle attachment. While on Medium speed, gradually add in the cubed softened butter. Beat until a thick and silky meringue buttercream.

TIP: If the mixture seems to thin, place back in the fridge for 20-40 minutes, then beat again until thickened.

Fold in the vanilla extract.

Generously brush each sponge cake layer with the coffee mixture on both sides.

Frost each cake layer with the Italian Meringue buttercream, then dust with cocoa powder to serve.

Store in a tall cake container at room temperature up the night before serving, then in the fridge up to 4-5 days.