Preheat the oven to 170ºC/gas 3. Line two large baking trays with greaseproof paper.
Sift together the flour, cocoa, baking powder, bicarbonate of soda and a pinch of sea salt.
Chop and melt the chocolate with the butter in a heatproof bowl over a pan of barely simmering water. Stir until smooth and set aside to cool.
In a large bowl, lightly beat the egg, then whisk in the sugars and soured cream. Stir in the melted chocolate and butter to combine.
Add the sifted dry ingredients and 5 tablespoons of boiling water, and mix until smooth. Fold in the white chocolate chips.
Spoon 12 even dollops onto each baking sheet, leaving plenty of space between them. Bake on the middle shelf of the oven for 10 to 12 minutes.
Leave to cool on the trays for a few minutes, then transfer to a wire rack to cool completely.
For the buttercream frosting, melt the white chocolate in a heatproof bowl over a pan of simmering water. Stir until smooth, then leave to cool.
Sift the icing sugar into a large bowl, add the butter and vanilla, then beat until smooth and light. Add the cooled chocolate and stir to combine.
Fill a large piping bag with frosting and pipe onto the flat side of 12 of the whoopies. Sandwich with the remaining cakes.
Melt all the glaze ingredients in a heatproof bowl over a pan of simmering water, stirring until smooth. Leave to cool slightly. Spoon over the pies, decorate with sprinkles, if you like, and leave to set before serving.