For the génoise fairy cakes, line a mini-muffin tray with 12 fairy cake cases and preheat the oven to 170C/150C(fan)/Gas 5.
Warm the bowl of a free-standing mixer by pouring in some boiling water and drying it with a tea towel.
Place the sugar and eggs in the warmed bowl and whisk at full speed for several minutes, or until the mixture is pale and doubled in volume.
While the mixture is whisking, melt the butter in a small saucepan. Sift the flour and mixed spice into a bowl.
Carefully add the melted butter around the edge of the mixture while still whisking.
Remove the bowl from the mixer and place on a tea towel, then sift and gently fold in the flour and mixed spice in two batches.
Transfer the mixture to a piping bag and carefully pipe into the cake cases. Bake for 10-12 minutes, or until golden-brown and springy to the touch.
Remove the cakes from the tins and cool on a wire rack. Peel the paper cases off the cakes when cooled.
For the plums, marinade the chopped plums in the orange liqueur in a small bowl. Set aside.
For the edible fairy cake cases, melt the chocolate in a heatproof bowl placed over a pan of simmering water. Set 12 more fairy cake cases in the mini-muffin tins.
Once melted, paint the chocolate inside the fairy cake cases and press the cooled genoise fairy cakes into each one, so the chocolate lines and coats the side of the cakes. Leave to set completely, then remove from the tins and remove the papers from the chocolate-lined cakes.
For the French meringue buttercream, combine the egg white and sugar in a medium-sized heatproof bowl set over a pan of barely simmering water.
Beat continuously with an electric hand-held whisk until the mixture is thick and glossy and all the sugar has dissolved.
Remove from the heat and continue to whisk until the mixture has cooled and is thick enough to hold its shape.
Beat the butter to a soft consistency and start adding small lumps of it to the meringue, a little at a time, whisking well after each addition.
Beat in the orange liqueur, orange extract and zest. Transfer the meringue buttercream to a piping bag fitted with a round nozzle and pipe it onto the cupcakes.
Drain the plums of their liqueur and carefully top each fairy cake with a plum piece and a sprinkling of edible gold glitter.