Set blender running and slowly add almonds, about half a cup at a time.
As they form a stiff paste in the bottom of the blender, stop and scrape them out into a bowl and continue until all the almonds are ground and in the bowl.
They should be as fine as fine breadcrumbs.
Break up any stiff mass that has formed, and add the sugar.
Rub the mixture through your hands, or with a pastry blender until uniform.
Remove any large pieces of unground almond.
Then sift in the confectioner's sugar and stir well .
In another bowl beat the egg with the brandy and add the almond extract if used.
Set the bowl over a pan of hot water, or cook in a double boiler, beating until the custard thickens slightly
Pour the custard onto the almond mixture in the bowl, and work the two together.
It may take a little time for the sugar to dissolve and the oils in the almonds to emerge, so that the whole can be worked into a smooth workable paste.
If there really is not enough liquid, you can add more brandy.
It should mould easily. If it is too soft to form a ball, you can add more almonds and sugar.
It is important at this point to wrap the ball of marzipan tightly in wax paper or cling film to stop it drying out.
Marzipan can be rolled out, using confectioner's sugar on the board and rolling pin, to stop it sticking.