This light and fluffy cake is perfect for a family gathering, or even to serve as a Christmas cake.
Preheat the oven to 180°C/160°C fan/350°F/Gas 4.
Make the sponges. Put the milk, syrup, treacle and sugar in a medium pan over a low heat, and heat until the sugar has dissolved. Set aside.
Mix together the spices and bicarbonate of soda in a small mixing bowl. Sift the flour into the bowl of a stand mixer fitted with the whisk attachment. Add the spice mixture. Add the softened butter and mix until the mixture resembles breadcrumbs.
Pour the syrup mixture into the flour mixture, along with the chopped ginger and the egg and mix well. Divide the mixture equally between the prepared cake tins and bake for 20 minutes, or until just firm to the touch. Invert the sponges onto a wire rack and leave to cool.
Meanwhile, make the gingerbread family. Put together the butter and sugar in the bowl of a stand mixer fitted with the whisk attachment. Whisk until pale and smooth, then add the salt, baking powder, bicarbonate of soda, and spices and stir to combine. Incorporate the egg yolk and treacle, then stir in the flour and mix to a firm dough. Cover the bowl with cling film and place in the fridge for 30 minutes.
Turn out the dough onto a lightly floured work surface. Cut off half the dough, wrap it in cling film and place it in the freezer for use another time. Roll out the remaining dough to about 5mm thick. Use the cutters to cut out 3 members of a gingerbread family, 1 Christmas tree and 4 hearts or stars, re-rolling the trimmings as necessary. Place the biscuits on the lined baking sheets and bake for 10 minutes, until golden. Remove from the oven, leave to cool on the trays for 5 minutes, then transfer to a wire rack to cool completely.
Make the buttercream. Place the egg white in the bowl of a stand mixer fitted with the whisk attachment. Set aside. Place the sugar and 60ml of water in a small pan over a medium heat. Slowly bring the water to the boil, swirling the pan from time to time to dissolve the sugar. Bring the temperature up to 118°C/244°F on a sugar thermometer.
Whisk the egg white on high speed to soft peaks, then, with the whisk still running, pour the sugar syrup down the side of the bowl in a continuous stream. Mix for about 15 minutes, until the meringue is glossy, holds stiff peaks, and has reached room temperature.
Keep the whisk running and add the butter a dice at a time, until incorporated and the mixture is thick and smooth. Then, whisk in the the cinnamon, ginger, lemon zest, and vanilla. Set aside.
Once the gingerbread biscuits have cooled, put the icing sugar and egg white in the bowl of a stand mixer fitted with the whisk attachment. Whisk at slow speed for 1 minute, and then at full speed for 5 minutes, until white and thick. Spoon the icing into the small piping bag fitted with the No.1 writing nozzle. Pipe features on the gingerbread family and decorate the tree and stars or hearts as you wish. Leave to set.
Use one third of the buttercream to sandwich the 2 sponges together, placing the uppermost cake top-side down to give you a neat, flat top. Spread the remaining buttercream over the top and around the sides of the cake. Arrange the Christmas tree and gingerbread family on the top of the cake, and the stars or hearts around the side.