In a small bowl stir together the double cream and soured cream. Cover and refrigerate overnight.
Let the soured cream mixture come to room temperature (it should take about 30 minutes). Preheat the oven to 240 C / Gas 9.
In a large bowl mix together the sugar, margarine and egg yolk together using an electric mixer. Stir the vanilla and bicarb into the soured cream mixture. Stir the soured cream mixture into the bowl with the sugar and margarine until well blended. Combine the flour and baking powder and stir into the mixture until fully incorporated.
Place the dough on a well-floured surface, as the dough will be sticky. Use ping pong ball-sized pieces of dough, and roll out into a 20 to 25cm rope. Form into a 'lazy eight' or 'infinity sign', and pinch the ends together. Place on ungreased baking trays.
Bake for 5 minutes in the preheated oven, or until slightly brown. Cool completely before storing in an airtight container. Store at room temperature for up to 5 days, or 1 month in the freezer.