Heat the oven to 160degC. Line a swiss roll tin with baking paper (about 20cm x 30cm).
Cream the butter and sugar together until light and fluffy. Add the eggs, one at a time, beating well between each addition.
Sift together the flour, baking powder and spices and add to creamed mixture. Fold in the dry ingredients.
Divide dough into two balls and put in fridge until firm enough to roll out.
Roll each half out to fit the tray. Place the first half on the tray and spread jam over it.
Carefully lay the other half on top. Prick in a few places.
Bake for 20-25 minutes at 160degC. Cool.
Sift the icing sugar into a bowl and add the butter. Use enough hot water to mix to a soft icing.
When just warm ice the slice and sprinkle with jelly crystals. Cut into squares.