Weigh and mix together the flours, xanthan gum, cocoa, baking powder, bicarbonate of soda and salt, making sure any lumps are broken down (TIP: Weigh into a large airtight container and shake vigorously). Set aside.
Base-line about 3 large baking sheets with baking paper.
Preheat the oven to 180 C/350 F/Gas 4.
Mix the milk with the white wine vinegar in a jug and leave to stand for about 10 minutes (the mixture will become lumpy 'buttermilk').
In a large bowl, beat together the sugar, butter and vanilla until smooth, pale and fluffy.
Add the eggs one at a time, beating well between each until smooth and combined.
Add and fold in the flour mixture alternately with the 'buttermilk' (about a third quantity at a time) until the cake batter becomes smooth and thick. Be careful not to over-mix.
Spoon the mixture into a large piping bag, fitted with a 1 cm round plain nozzle.
Pipe onto the lined baking trays as round, slightly flattened mounds (roughly equal in size and leaving a gap for a little spreading between each), about 3 to 4 cm in diameter. (Or dollop on to the baking paper with a spoon).
Bake for 8 to 10 minutes until the tops are firm to touch (and a skewer inserted comes out clean).
Allow to cool slightly on the trays, before transferring onto a wire rack to cool completely.
Beat the softened butter in a large bowl until creamy and light.
Gradually add and beat in the icing sugar a little at a time, with a drop of milk as needed if the mixture becomes too stiff.
Add a few drops of mint extract a little at a time to taste and green food colour (optional) and beat to blend.
When you have a thick but spreadable frosting, pair the whoopie cakes for size.
Spread or pipe a good layer of frosting onto the flat side of one of the whoopie cakes and sandwich together with its other side.
To decorate (optional)
Melt the chocolate either in a glass bowl over a pan of just simmering water (stirring) or in the microwave at medium setting and using 30 second bursts, stirring between each until smooth.
Either lightly part-dip the whoopie pies in the chocolate, or drizzle the chocolate over them and sprinkle with chocolate sprinkles, etc.
Set to one side on baking paper to allow the chocolate to set.
* Note: nutritional information is an estimate & may vary according to portion size/ingredient variants.Gluten Free Alchemist Flour Blend B can be found on the GFA Flours & Flour BlendingPage. If using an alternative flour (particularly one which is heavily rice-based), the texture will be different and more liquid may be required.