1-Set out two bowls: one containing the softened butter, the other holding a fine-mesh sieve.

2-Combine frozen raspberries, lemon zest, and lemon juice in a small saucepan. Cook over medium-low heat until the raspberries are soft and mostly liquid.

3-Press the cooked raspberry mixture through the fine-mesh sieve into the empty bowl to remove seeds and zest. Scrape the sieve bottom to collect the juice (you should have about ¾ cup). Discard the solids. Rinse and dry the sieve, then place it over the bowl containing the softened butter.

4-Return the strained raspberry juice to the saucepan. Add the granulated sugar and stir. If the juice mixture is hot, allow it to cool slightly.

5-Whisk the eggs and the optional tapioca starch or cornstarch into the raspberry-sugar mixture in the saucepan.

6-Cook the mixture over medium heat, stirring constantly, until the curd thickens significantly and coats the back of a spoon.

7-Immediately pour the hot curd through the sieve placed over the butter. Press firmly with a spatula to extract as much curd as possible, removing any cooked egg bits.

8-Stir the hot curd vigorously into the softened butter until the mixture is completely smooth and shiny.

9-Transfer the finished curd to a clean jar or container. Chill thoroughly before storing in the refrigerator.