Fill a small saucepan with 1–2 inches of water and bring it to a gentle simmer. Place a heatproof bowl over the saucepan, making sure the bottom of the bowl doesn't touch the water.

Add the frozen passion fruit cubes to the bowl and whisk until completely melted.

Whisk in the sugar until dissolved, then add the butter and continue whisking until fully melted and smooth.

Reduce the heat to low.

In a separate bowl, whisk the egg yolks until smooth. Slowly add a few tablespoons of the warm passion fruit mixture while whisking constantly. Repeat this a few times until the egg yolks feel warm. This process, called tempering, gently raises the temperature of the eggs so they don't scramble.

Slowly pour the tempered egg mixture back into the bowl over the double boiler, whisking constantly.

Continue whisking over gentle heat until the curd thickens enough to coat the back of a spoon. To test, run your finger through the curd on the back of the spoon. If the line stays clean without the curd running back together, it's ready.

Remove from the heat and let cool for 5–10 minutes. Strain through a fine-mesh sieve for an extra silky texture.

Press plastic wrap directly onto the surface of the curd to prevent a skin from forming. Transfer to an airtight container and refrigerate until completely chilled before using. Store refrigerated for up to 1 week.