Remove the pan from the heat and beat in the spices and bicarbonate of soda, followed by the flour. Mix until it forms a smooth dough then cover the pan and place in the fridge for about an hour, until the dough is firm.
Preheat the oven to 180°C/350°F/gas mark 4 and line two baking trays with baking parchment. Bring the dough together into a ball and roll out on a lightly floured surface to about 3mm (1/8) inch thick.
Place the trays in the freezer for 15 minutes until firm then bake for 10-15 minutes until the biscuits feel firm when lightly pressed and have begun to darken a little around the edges. The smaller ones will cook quicker than big ones so try to group similar sized ones together on the trays.
Once the biscuits are cool, make the royal icing. Sift the powdered sugar into a bowl and add the lemon juice. Stir in the egg white a little at a time until the icing is smooth and thick but pipe-able; if you drizzle some back into the bowl the pattern should disappear after about 8 seconds. If it is too thick then add a little more egg white (or lemon juice if you've run out), and if it is too runny then add a little more sifted powdered sugar.
Spoon the icing into a piping bag fitted with a 1mm plain round nozzle. Pipe snowflake designs onto the biscuits (Pinterest is a great place to get inspiration for patterns!) and set them aside for half an hour to dry. Sprinkle with edible glitter if using.
Trim the top of the cake if it is domed and place it upside-down on a cake board or serving platter so that the flat bottom is now the top. Dust a worksurface with icing sugar and roll a bit of the marzipan into a length of rope. Press the rope into the gap between the bottom of the cake and the board so that it is flat all the way round.
Gently heat the apricot jam until it is runny then brush it all over the cake. Carefully lift the marzipan circle over the cake and press it down the sides so that the cake is completely covered. You will probably get some folds and creases, just trim off the excess marzipan and smooth the edges together, it doesn't need to be perfect. Trim off the excess marzipan from around the bottom of the cake.
If you have time, allow the cake to dry out for a couple of days before icing to prevent any oils from the marzipan seeping into the icing. If not then you can go right ahead and ice it.
Place the egg whites in a large bowl and whisk until frothy. Add the icing sugar a little at a time and fold in. Stir in the lemon juice and glycerine if using (the glycerine prevents the icing from setting tooth-breakingly hard).
Beat the icing with an electric mixer until it is very stiff and white and forms stiff peaks.
Use a palette knife to spread the icing all over the cake, there is no need to get it too smooth.
Arrange a pile of gingerbread stars on top of the cake and add a couple of small ones around the sides. Sprinkle with edible glitter and set aside for a couple of hours to firm up. Store in an airtight container.