Preheat oven to 350ºF. Prepare three 5x2" pans, by brushing or spraying non-stick, and place them on a baking sheet.
In the bowl of a stand mixer, fitted with the paddle attachment, cream the butter, then add the sugar and cream until fully combined.
Scrape the beans of two vanilla pods into the eggs and lightly whisk. Add the vanilla-egg mixture to the creamed butter in two additions, beating well after each addition. Scrape down the sides of the bowl and continue to beat until light and fluffy.
Sift the flour, baking powder and baking soda together and stir in the salt.
Turn the mixer to low and add the flour mixture to the butter mixture in three additions, alternating with the sour cream and starting and ending with the flour mixture. Scrape down the sides of the bowl, as needed.
Transfer the batter to each pan in even amounts, filling pans about two-thirds.
Return pans to baking sheet pan and bake on the middle rack for 25 to 30 minutes, or until a toothpick inserted in the middle comes out clean.
Remove cakes from oven and allow to cool for a few minutes on a cooling rack before turning out. Cool completely before decorating.
To make the buttercream:
In a medium saucepan, add the water and the first measure of sugar and cook without stirring until the mixture reaches (116ºC/240ºF), or what is labelled as 'soft ball' on your thermometer.
While the sugar is cooking, add the egg whites to the bowl of a stand mixer fitted with the whisk attachment and set aside.
Once the sugar syrup reaches 110C/230F, start whipping the eggs on medium speed. When the egg whites begin to appear foam, sprinkle in the second measure of sugar while whipping.
The egg whites should be at soft peaks by the time the sugar reaches 116C/240F/soft ball.
With the egg whites whipping on medium speed, rest the lip of the pot on the lip of the mixing bowl and pour the syrup into the whipping egg whites in a slow, steady stream down the side of of the bowl.
Continue to whip until the mixture has cooled to room temperature and the meringue is full and billowy.
On medium speed, add the butter in small chunks and the vanilla beans, continuing to whip until the smooth and light.
To make the syrup:
heat sugar and water in a small saucepan, until sugar dissolves.
Remove from heat and stir in passion fruit puree.
Chocolate drip:
Melt chocolate in 20 second intervals in a microwave and stir until smooth.
Add colour and stir until uniform and transfer to a piping bag or plastic bottle fitted with a cone nozzle.
Assembly:
Assemble as below and go to town with decorating it to your liking. It's the best part!