In a mixing bowl, beat together the 125g of butter, ½ cup of brown sugar, ½ cup of caster sugar, and 1 teaspoon of vanilla extract until pale and creamy.
Add in 1 egg and beat to combine.
Add in 1 teaspoon of baking soda, ¼ cup of cocoa powder, and 1 teaspoon of red food dye, and mix together.
Gradually add in 2 ¼ cups of plain flour and 1 ⅓ cups of buttermilk, mixing until just combined.
Scoop the batter onto baking trays and bake at 190°C for 10 minutes.
Allow the whoopie pies to cool completely on a wire rack.
In a mixing bowl, beat together 125g of cream cheese, 50g of butter, and 1 teaspoon of vanilla extract.
Gradually add in 2 ¼ cups of icing sugar and ½ tablespoon of milk, mixing until smooth.
Spread the cream cheese filling onto the bottom half of the cooled whoopie pies, then top with the remaining halves.
