Set out two bowls, one with the butter in it and one with a fine mesh sieve over it.
Place the frozen raspberries, the zest and the lemon juice in a small pot. Cook on medium to low heat until the raspberries soften and burst and it’s mostly liquid.
Press the raspberry mix through the fine mesh sieve to remove the seeds and zest, scraping the bottom of the sieve. You should have ¾ cup raspberry lemon juice. Discard the seeds and rinse the sieve and pat dry, and set it over the bowl with the butter in it.
Return the raspberry juice to the pot and add the sugar and stir. The mix should have cooled quite a bit by now, if it’s hot, wait a bit. Whisk in the eggs (and starch if using) then return the pot to the stove, cooking at medium heat and whisking until the curd is thick at the bottom and covers the back of a wooden spoon or rubber spatula.
Pour the curd through the sieve and press to get as much curd out as possible, scraping the bottom of the sieve. Stir the curd and the butter until the butter is fully melted and you have a shiny, smooth curd. Transfer to a mason jar and chill, then store in the fridge. The curd will become more tart-tasting and stronger in flavor as it chills.