Soak the pistachios. Submerge pistachios in a container of water and let the nuts soak for 6-12 hours or overnight. You can stick it in a tupperware in the fridge if you'd like.
Drain and peel. Drain the pistachios, then rub them vigorously between two layers of a clean dish cloth to help loosen the brown papery skins. Some will come off on the towel, and the rest should come off easily by hand. Put on a show or something while you remove the skins from each pistachio; this is the longest step and takes about 20-30 minutes. This step is technically optional, but you will end up with more of a brown syrup if you leave the papery skins on.
Blend. Add the pistachios to a blender with 1 cup of water. Blend on low until the pistachios are mostly incorporated, then turn to high speed for 1-2 minutes until the mixture is as smooth as possible.
Strain the pistachio milk. Lay a double layer of cheesecloth in a fine mesh sieve and place it over a 2-cup measuring cup or bowl. Let the milk drain until you have a cup of smooth green liquid. You can gather the corners of the cheesecloth and gently squeeze to encourage the milk to come out, but do not squeeze so hard that pistachio pulp starts oozing out of the cheesecloth. Set aside the pistachio pulp for later if you plan to make pistachio crunch topping; do not discard.
Add the sugar. Add pistachio milk and sugar to a small saucepan and heat over medium, stirring frequently, until the sugar is dissolved. Remove from heat, cool, and add almond extract. Store in an airtight container in the fridge for up to two weeks.
Enjoy! Add to lattes, drizzle over ice cream, or use to make Italian sodas!
Notes
Nutritional values are an estimate based on 50% of the nutritional content of the pistachios being strained out.Almond extract: In my taste tests, adding a small amount of almond extract gave cold foam flavored with this syrup a more pronounced pistachio flavor; more on par with the boldness of commercial pistachio syrup, but much less artificial tasting.