Preheat the oven to 180C/350F/Gas mark 4
Sift the flour, baking powder and salt into a mixing bowl. Rub in the margarine until the mixture resembles fine breadcrumbs, then stir in the sugar. Add the vanilla essence, then bind to a pliable dough, using about 1-2 tsp. water.
Divide the dough in half. Return one half to the mixing bowl. Sprinkle the carob over the top and work it in evenly, using a fork. Roll out the plain dough on a lightly floured surface to a rectangle measuring 7 x 9 inches. Set aside. Roll out the carob dough to the same size and place on top of the plain dough. Roll up both pieces from a long side, like a Swiss roll, keeping the join underneath. Chill.
Cut the chilled dough roll into slices ½ inch thick. Reshape into neat rounds with hands, and place well apart on the prepared baking trays. Bake for 15-20 minutes until the plain part of each Catherine wheel is golden brown. Makes 24