Using the back of a large spoon cream 125g butter until it is light. Mix in the icing sugar then 1/4 tsp. vanilla. Now work in the flour. Knead the dough on an unfloured surface until it is smooth.
Spray a pan very lightly with a non-stick spray. Put the ball of dough in the middle of the pan, and working from the inside out firmly press the dough into the pan. Prick the entire surface with a fork, and bake at 170 degrees C for 30-35 minutes or until lightly browned.
Let shortbread cool in its pan for 10 minutes before you loosen the edges with a knife and flip the pan upside down onto a cutting board. If shortbread does not come out right away, tap one edge of the pan against the board. Do not cut!! Allow shortbread to cool.
Cut strips of greaseproof paper and lay them over the edges of the shortbread pan, being sure to cover the rim completely. Return baked shortbread to pan.
To Make the Toffee: Mix the 7/8 cup sugar, cream of tartar and cream in a small heavy saucepan. Bring to a boil over med-high heat, stirring constantly, and cook for 2-3 minutes. Add the butter.
Continue to cook and stir the toffee until it turns a pale brown and reaches the soft crack stage. One way to test it is to drop a small amount of molten toffee into a cup of ice water. If it hardens to a crunchy blob the toffee is ready. Take the pan off the heat and let it cool. Stir in 1/2 teaspoon vanilla.
Now pour the toffee onto the shortbread. Use as much of the toffee as you like. Let the toffee cool until it is just warm to the touch. Turn the shortbread onto a cutting board and peel off the greaseproof paper strips. Using a serrated knife slowly cut the shortbread into serving pieces. Turn the pieces toffee side up to cool completely.
To Make Top Chocolate Layer: Melt the chocolate in a small pan over a tiny flame. Cool it slightly then spread a layer of chocolate on top of the toffee layer on the shortbread.
Place the finished biscuits chocolate side up on a plate and place in the refrigerator to finish hardening. Remove shortbread 1/2 hour before you plan to eat it.