Pre-heat oven to 190ºC (fan forced). Spray two baking trays with non-stick spray.
Into the bowl of an electric mixer, add butter and sugar and mix until fluffy (about 3 minutes). Add eggs, one at a time mixing until combined after each addition. Remove bowl from the stand and with a rubber spatula, stir in buttermilk and vanilla extract.
In a medium bowl, combine flour, cocoa powder, baking powder and salt. Add to the batter and mix well.
In a separate bowl, mix together the hot water and bicarb soda until dissolved. Add this to the batter and mix until well combined.
Onto the prepared tray, scoop heaped tablespoons of batter, leaving 3cm between each to allow for spreading whilst baking. Bake in the oven for 8-10 minutes.
Remove from oven and allow to cool on the tray for just a few minutes before transferring them to a wire rack to cool completely.
Prepare filling: into the bowl of an electric mixer or hand held beater, add the thickened cream, the cream stabilizer, icing sugar and vanilla. Beat for 4 minutes until thick and fluffy.
Place a large spoonful of cream into a piping bag and pipe onto half of the whoopie pies and top each one with another pie.
If desired, dust with icing sugar.
To make your own buttermilk: In a measuring jug add 1 tablespoon of white vinegar and make up 250ml with milk. Stir and let sit for 5-10 mins, then use in the recipe as directed.