Heat oven to 160C/fan 140C/gas 3. Butter and line a 20cm round cake tin (about 8cm deep) with baking parchment. Put dates in a bowl and cover with boiling water. Heat the butter in a small pan until melted, and stir in the sugar. Allow to cool slightly.
Beat in the eggs, ginger and lemon zest. Drain the dates and chop them finely. Scrape them into the pan too, and mix well. Stir in the flour, then apple. Spoon into the cake tin, put the tin on a baking sheet (this stops the base browning too much) then bake for about 1¼ hrs, until well risen. A skewer stuck into the cake should come out with a few moist crumbs sticking to it. Leave it to cool in the tin.
Break the white chocolate into a non-metallic bowl. Heat in the microwave, on medium setting, in one minute bursts until melted (or in a bowl over a pan of simmering water). Remove the cake from the tin and peel off the paper. Trickle the chocolate over the cake, scatter with the crystallised lemon peel and coffee crystals and leave to set before serving. If well wrapped, the cake keeps for at least a week – it just gets better and better. Or, wrap the cooled cake in several layers of cling film before decorating, and freeze for up to two months.