Soften the butter and cream with the sugar. Add the vanilla essence, then beat in the eggs, one at a time, beating well between each addition. Sift the flour and salt together and fold in the ground almonds. Work into the creamed mixture to form a dough, then knead lightly, and wrap in plastic wrap. Chill for 1 hour. Divide the dough into pieces about the size of a walnut. Lightly flour a board and roll out each little piece of dough into a sausage shape. It should be about 20cm long and the thickness of your little finger in the middle, but thinner at each end. Twist each piece into a pretzel shape, like a loose knot, and press the ends firmly together to make a double ring. Arrange the biscuits on greased baking sheets and bake in a hot oven 200C (400F) for 10 to 12 minutes, or until pale golden. Place on wire racks and dredge thickly with icing sugar while still hot. Cool, and store in an airtight tin. Just before serving, sift more icing sugar over the biscuits.