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1
Preheat the oven to 170C (325F), Gas Mark 3.
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2
Line 2 baking trays with baking paper.
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3
Heat the milk gently in a small saucepan, add the sugar, then take the pan off the heat, add the soured cream and stir through to combine.
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4
Leave to cool to body temperature.
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5
Meanwhile, put the flour, bicarbonate of soda, baking powder, cocoa and salt together in a large bowl.
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6
Once the milk mixture has cooled to body temperature, add the eggs, vanilla and oil to the pan and stir to just combine.
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7
Pour the liquid ingredients into the flour mixture and fold gently together.
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8
Transfer the mixture into a piping bag fitted with a large nozzle and pipe circles about 8cm (3in) in diameter on to the lined baking trays, spaced about 8cm (3in) apart.
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9
Bake in the oven for 8-10 minutes, or until a skewer inserted in the thickest part comes out clean.
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10
Leave the whoopie cakes to cool on the trays.
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11
Meanwhile, to make the buttercream, beat the butter, icing sugar and vanilla together in a bowl until light and fluffy, then beat in the milk.
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12
Once the whoopies are completely cool, gently release them from the baking paper.
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13
Take a generous dollop of the buttercream and spread it onto the flat base of one of the whoopies, then sandwich it together with a second whoopie.
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14
Repeat with the rest of the cakes.
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15
Variations Vanilla: Omit the cocoa powder and add an extra 55g (2oz) of flour.
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16
Coffee: Omit the cocoa powder and add an extra 55g (2oz) of flour, plus a few drops of coffee essence or 2 tablespoons of instant coffee powder stirred into the warmed milk.
