Welcome to Nigella

Anyway, the way you make it is:

1. Put the biscuits into a plastic bag or similar and whack repeatedly with a rolling pin. You want some very fine crumbs and some bite sized pieces. Some will also break up a bit more with the stirring so don't bash them up too much.
2. Melt the butter and syrup together in a pan and break up the chocolate and put that straight into the butter/syrup. You don't need to worry about heating it over water or anything as long as you don't have the heat up too high because the butter and the syrup will prevent it burning.
3. Once it's all melted together into a smooth, silky sauce add the crushed digestives and stir them in until it's all well mixed. The little crumbs should soak up the chocolate sauce while the larger bits get coated with it. You may not need all the biscuits but you want it so that there is hardly any obvious chocolate sauce, but the biscuits are still glistening and coated with the sauce.
4. Finally pour the biscuit mixture into a greased or lined spring form cake tin and use a potato masher to squash it all down. The aim isn't to crush the biscuits but to pack it all together so that the chocolate sauce bonds the whole lot together. Then put it into the fridge to cool.
5. Once it's mostly cool, but not completely, decorate with chocolate buttons (it needs to be just warm enough for them to melt slightly and stick but not so warm they just lose their shape). Return it to the fridge until it's completely cold.