Preparation

• Purée the blackberries with 1/2 cup water and the lemon juice in a blender until completely smooth. Strain through a fine-mesh sieve set over a large bowl. Rinse the blender and sieve. Refrigerate the purée until very cold, about 30 minutes. (You can refrigerate the purée up to 1 day).
• Put the sugar and 1 cup water into a medium container. Cover tightly and shake until the sugar is dissolved. Add the corn syrup; stir to combine. Chill in the refrigerator until cold, about 30 minutes.
• Put the blackberry purée, 1 cup of the simple syrup, the elderflower liqueur, and guar gum (if using) in a blender. Blend until smooth. Strain the mixture through a fine-mesh sieve into a tall (6 to 8 cup) container.
• To check the density of the sorbet base, gently lower the egg into the container with a slotted spoon (don’t drop it in or it could break). If the egg sinks, remove it and stir in 2 Tbs. of the simple syrup. The goal is to adjust the sugar density with the syrup until the egg floats just below the surface of the sorbet base with an exposed area of shell that’s about the size of a quarter. Keep testing with the egg, adding more syrup 2 Tbs. at a time, until the egg floats as shown below. When it does, remove the egg. Refrigerate the sorbet base until very cold, at least 30 minutes.
• Freeze the sorbet base in an ice cream maker according to the manufacturer’s instructions. While the sorbet is churning, put two pint-size containers in the freezer. Transfer the sorbet to the pint containers and freeze until hardened, at least 4 hours. If the sorbet hardens too much to scoop, let it sit in the refrigerator for 20 minutes before serving.

Make Ahead Tips

You can make the syrup up to 2 weeks ahead and store, covered, in the refrigerator.

You can refrigerate the sorbet base for up to 1 day before freezing.

The finished sorbet will keep in the freezer for up to 2 weeks; after that the texture may become icy, but you can thaw and refreeze the base in your ice cream maker.