In the container of a blender, process cherries, 6 tablespoons (90 grams) water, and lemon juice until smooth.

In a medium heavy-bottomed saucepan, cook cherry mixture, hibiscus leaves, and ¼ cup (50 grams) sugar over medium heat, stirring frequently, until liquid begins to thicken and bubbles form around sides of pan, 3 to 4 minutes.

In a medium heatproof bowl, whisk together egg yolks, cornstarch, salt, and remaining ¼ cup (50 grams) sugar until smooth. Gradually add hot cherry mixture, whisking constantly until well combined. Pour egg yolk mixture into pan, and cook over medium heat, whisking constantly, until mixture is thickened, 5 to 6 minutes.

Strain mixture through a fine-mesh sieve into a heatproof bowl, discarding solids. Whisk in butter until melted and smooth. Cover with plastic wrap, pressing wrap directly onto surface of curd to prevent a skin from forming. Let cool completely. Use immediately, or cover and refrigerate for up to 3 days. Whisk curd before using.