|350 g||sugar (divided)|
|345 g||butter (softened, divided)|
|2 tsp||vanilla extract (divided)|
|1¼ tsp||baking soda|
|½ tsp||fleur de sel|
|fleur de sel for serving|
|butter for greasing|
Preheat the oven to 180°C/350°F. Thoroughly combine just over half of the sugar and molasses with a fork to make brown sugar. In a large bowl, cream together one third of the butter and all the brown sugar with a hand mixer with beaters. Add the whole egg and half the vanilla extract.
In a separate bowl, whisk together the flour, cocoa, baking soda, and salt. Add the dry ingredients to the butter-sugar mixture in three parts, alternating with the buttermilk, until everything is combined.
Using an ice cream scoop, scoop out mounds of batter onto a greased baking sheet. Leave plenty of room around each mound, as they will expand outwards as they bake. Bake at 180°C/350°F until tops are springy, approx. 12 – 15 min. Remove from oven and let cool completely or for at least 1 hr.
Meanwhile, make the buttercream filling. Set a pot with some water over medium heat and set a heatproof bowl over the pot once the water begins to simmer. Combine egg whites and remaining sugar and whisk constantly until sugar is dissolved and egg white mixture reaches 70°C/160°F on a candy or food thermometer. Remove from heat.
Using a hand mixer with beaters, beat the egg white and sugar mixture on high until it doubles in volume and becomes thick. Continue beating until the mixture is cool to the touch. Reduce the speed and add remaining butter in small chunks, until all the butter is mixed into the buttercream. Deseed the vanilla bean and add the seeds, remaining vanilla extract, and fleur de sel to the buttercream. Transfer to a piping bag with a round tip.
Pair similarly sized cakes together. Arrange the bottoms with the flat side facing upwards, and pipe the buttercream onto them, leaving a little room around the edges. Sprinkle with fleur de sel, and top with the other cake half. Wrap individually in plastic wrap and store in the fridge for up to a week. Individually wrapped and stored in a resealable freezer bag, they will last for up to 3 months. Enjoy!