Can I make lemon curd more sour/tart after it's cooked? I prepared a batch of lemon curd using an unfamiliar recipe and it just doesn't have what I'm looking for in terms of tartness and lemon flavor. I'd hate to waste it, so I'm thinking of zesting and juicing more lemons to add to the prepared curd - but I'm worried this will ruin the emulsion!
Is there a way to salvage this batch and get the tartness I'm after?
For the record, the recipe I'm starting with uses the following ingredients:

3/4 cup sugar
2 Tablespoons cornstarch
1/8 teaspoon salt
3/4 cup cold water
2 egg yolks, slightly beaten
Juice of 1 1/2 lemons
2 teaspoons lemon zest
1 Tablespoon butter

Going purely by taste, I'm a good 2-3 juiced lemons away from where I'd prefer to be.
I have used both citric acid (food grade, sold for canning and jam making, not the descaler) and ascorbic acid (vitamin C) in similar cases when only the acidity of a dish was insufficient. Sometimes lemons are just not sour enough.
The powder comes in very fine texture and can even made finer with a mortar and pestle. A very small amount will go a long way and that’s why it won’t affect the ratios of a dish and a can be stirred into even thick preparations like lemon curd. You can gently warm up your curd so that the butter in it softens, if necessary.