Place the egg yolks in the bowl of a stand mixer with the paddle attachment and beat on medium high speed for 3 to 5 minutes until fluffy and pale yellow.

In a small saucepan set over medium heat, stir together the water and sugar. Cook until mixture becomes a clear syrup and the temperature reaches 240 degrees F (115 degrees C). After the initial stirring together of water and sugar, do not stir again while the mixture is cooking. Remove the syrup from the heat.

Turn the mixer to medium speed and pour the syrup into the bowl in a slow stream. Then, turn the mixer on high speed, and beat until the mixture is fully cooled, about 10 minutes. It is very important to make sure the mixture has cooled before moving onto the next step. It may take longer than 10 minutes.

Lower the mixer speed to medium and slowly add the butter, 2 tablespoons at a time, allowing each new addition of butter to incorporate into the mixture before adding more. Once all of the butter has been incorporated. Beat on high speed for 2-3 minutes, until the mixture emulsifies into a smooth and creamy buttercream. Don’t worry if it looks curdled before it has emulsified. It will come together.

Sift the cocoa powder and add the salt and vanilla bean paste into the buttercream. Mix just until fully combined. Spread the buttercream on top of the cake, slice, and serve. The cake is most delicious the day that it is made but can also be stored in the refrigerator for 3 days.