-
1
Heat the oven to 175°C. Line two sheet pans with baking paper. Whisk together the flour, cocoa powder, bicarbonate of soda, and salt in a bowl.
-
2
In a second bowl, whisk the sugar, oil, and eggs until combined. Add the buttermilk and vanilla extract and whisk again. Fold the dry ingredients into the wet until no streaks of flour remain — the batter will be thick.
-
3
Scoop the batter onto the prepared pans in level rounds of approximately 8 cm diameter and 2 cm thickness, spacing them well apart. Bake at 175°C for 10–12 minutes. This is the step on which the dish lives or dies: the rounds must emerge soft to the touch — a finger pressed gently into the centre should meet only the lightest resistance. An overbaked round will harden on cooling and the cake-like character that defines the format is lost.
-
4
Transfer the rounds to a wire rack. Allow them to cool completely before filling — warm cake melts the filling and the sandwich collapses.
-
5
Beat the butter and marshmallow fluff together until light and noticeably increased in volume. Add the sifted icing sugar and vanilla extract and beat again until the mixture is fully white and fluffy — it should hold a soft peak and feel airy on the spatula.
-
6
Match the rounds into pairs of equal size. Spread or pipe a generous, thick layer of filling onto the flat side of one round of each pair and press the second round on top, flat side down. The white filling should be visible at the perimeter — this dark-and-white contrast is the visual signature of the finished piece.