Whip the margarine, butter and icing sugar together well. You can use all butter for sure, don't get me wrong, and then it is a shortbread for the purist. I like it half and half. Once it looks all deliciously creamy, resist from eating it by the spoonful, and get your flour ready.
Start the beaters, and slowly add in the flour, until it has all been added in. Then you can proceed to let your mixer do all the work for you and whip it for about 5-6 minutes. You are wanting lighter than air shortbread here.
Drop by rounded teaspoon onto a baking sheet lined with parchment paper, and add whatever you want on top. I usually just go for a piece of cherry, but sprinkles are excellent as well. The batter is perfect for cookie presses as well, I have had fantastic luck using this recipe. Just whip it about half the time to give them more density to hold the shape, otherwise they just break off in little air puff pieces.
Bake them in a 250 degree oven for about half an hour, you want to dry out the shortbread in essence, not bake it, thus the low temperature.