Lightly grease two baking trays.
Cream the fat with the sugar until very soft, light and fluffy.
Sift together the flour, bicarbonate of soda and cream of tartar, then work these dry ingredients into the creamed mixture to make a soft dough.
Divide the dough into 28 and form each piece into a ball.
Place at intervals on the prepared baking trays and press a pecan nut in the centre of half the quantity of biscuits.
Bake in a moderate oven (180 C, 350 F, gas 4) for 20 minutes, or until lightly browned.
Remove the hot biscuits from the trays and transfer them to a wire rack to cool completely.
While the biscuits are cooling, make the butter icing according to the recipe instructions.
Finely crush the nut brittle with a rolling pin, add it to the butter icing and beat well.
Sandwich the biscuits together in pairs with the filling.