In a food mixer, beat the margarine and sugar together very well (for at least a minute), then add the rice flour and salt. Beat again.
Add the plain flour and mix on a very low speed until just combined into a crumbly dough (it won't really hold together at this stage, don't panic).
Tip out onto a floured surface, and use your hands to press the dough together into a large lump. Divide the dough into two equal halves, and return one to the bowl. Sprinkle this half with the lemon zest and poppy seeds, and gently knead it together until evenly distributed, (avoid over-mixing, do as little as you can).
Add the chocolate chips and chopped macadamia nuts to the other half of the dough, and knead gently until evenly distributed.
Cover two baking trays with baking parchment or greaseproof paper. Roll out the dough to around half a centimetre thick, (a little too thick is far better than too thin where shortbread is concerned!). Use a heart shaped cutter to cut out as many biscuits as you can, placing them on the baking trays. Then roll the offcuts together and re-roll and cut again. Continue until all the dough has been used up.
Repeat the process with the lemon & poppyseed dough. Place both trays in the fridge for an hour.
Whilst the biscuits are chilling, preheat your oven to 160°C / 320°F / Gas Mark 3. When an hour is up, remove them from the fridge, sprinkle each one with caster sugar and pat gently to press it into the dough.
Bake the biscuits for 20 minutes, they should remain fairly pale, don't let them turn too brown. After 20 minutes check underneath a few - if they are still a little soggy, flip them all over and return to the oven for a further 3-4 minutes.