In a medium bowl, whisk together the sugar, optional yuzu zest, egg yolks, and cornstarch until smooth.

In a small saucepan, heat the yuzu juice on medium to medium-high heat until simmering. Turn the heat down to medium low.

Very slowly whisk the heated yuzu juice into the egg mixture to temper it. Pour the mixture back into the saucepan, place on the stove over medium low heat, and cook, whisking constantly, until mixture thickens and becomes pudding-like. This should only take 5 minutes or so.

If you used the optional zest, pour the cooked pastry cream through a fine sieve into a medium bowl. (If you don't care about bits of zest in the cream, or didn't use the zest, you can skip the sieve.) Whisk in the butter, one piece at a time, waiting until each piece is fully incorporated before adding the next one. The mixture should be very smooth and shiny. Cover the pastry cream with plastic wrap, placing the wrap directly on the surface to keep a skin from forming. Refrigerate for at least 4 hours, or overnight.