Gluten free lemon bars

Recipe from Noshtastic.  I subscribed.

SHORTBREAD:
175g (Approx. 1.5 sticks) Dairy Free Margarine (or butter if you don't need to avoid dairy)
75g (1/3 cup + 1 tbspn) sugar
250g (Approx 1.5 cups (see notes!) ) gluten free all purpose flour
1tsp xanthan gum (omit if your flour has any gums added)
½ tsp gluten free baking powder
TOPPING:
300g 1½ cups sugar
120mls ½ cup fresh lemon juice
4 large eggs, beaten
30g ¼ cup all purpose gluten free flour, or cornstarch
gluten free powdered sugar to dust

Instructions

*Please read the notes first!*
You will need a 9x13 baking pan, line the base and sides with parchment paper
Preheat your oven to 350f
Cream butter (or margarine) and sugar in mixer.
In separate bowl add flour, xanthan gum, baking powder and mix well. Omit the Xanthan gum if you are using a ready made flour mixture which has gum added.
Add flour mix to butter/sugar and mix well until combined.
Your 'dough' may be soft, especially if you used dairy free spread. If you find it is super soft then put it in the fridge for half an hour to firm up a little
Press your dough into the lined baking pan and spread out evenly with an offset spatula, I sometimes find that the back of a spoon works well too.
It's hard to tell when looking at the baking pan, but try and get the dough spread with an even thickness.
Bake at 350 for 20 mins or until lightly brown.
While base is cooking make the filling, mix all the topping ingredients except the powdered sugar. When base is cooked pour over topping and return to the oven to bake for a further 30 minutes or until the center is set and no longer moves when you shake the pan.
Cool for two hours in the fridge, dust with icing sugar when fully cooled and cut into squares.

Notes
1. I bake by weight as I find that gives the most consistent results, especially when baking gluten free. One cup of your gluten free flour blend may not weigh the same as my blend, all the commercially available blends have different mixes and ratios of flour in them. I’ve weighed cups of different blends of flour and noted that a cup of gluten free flour can vary in weight from 125g up to as much as 170g depending on how you scoop it, and what type of mix you use. You can see how this could drastically alter the outcome of a recipe.

I recognise that most Americans are used to the cup method which is why I also include those measurements,but they may not give you accurate results. If you are a keen gluten free baker I would encourage you to invest in a scale, you can get one for only $10 - $15.

2.I prefer to use my own homemade gluten free flour blend as it's cheaper and I get really great results.

3. If you can, avoid buying flour with gums already added. The reason for this is that you have no way of knowing how much gum is in there and that also can alter the outcome of the recipe. So for example, too much gum can give you a more sticky texture in a cupcake. And though the cupcake recipe will still work, it won’t be as nice as if you measured and added the gum yourself.

4. One potential problem with most lemon bars recipe is that the liquid lemon mixture can sometimes make the shortbread soft on the top, though not all the way through. I've found that if you have the lemon mix ready to go straight on the shortbread as soon as it's cooked and then quickly get it back in the oven, that helps a bit. Secondly, if you let the lemon bars fully cool and then chill in the fridge the shortbread base will crisp up nicely. The only sure fire way to avoid this issue is to make a kind of lemon curd mixture for the topping, but that's more involved and time consuming and requires cooking it on the stove top first so I've just stuck with the traditional way of doing it.

5. If you have to use Dairy free margarine as I do please make sure you find one that is suitable for baking. Some margarine brands have a lot of water and will be sure to disappoint if you try to bake with them, I typically use Earth Balance.