Choose your fave flavor of ice cream. Premium ice creams are best for thick milkshakes because they tend to be heavier and contain less air than cheaper ones. Avoid those with artificial thickeners like guar gum or modified food starch.
Put ice cream and cold milk or nut milk into the blender. Add syrup, if using, last so it mixes in well instead of sinking to the bottom of the blender.
Turn the blender on to the lowest setting and blend the shake until smooth.
Open the cover of the blender to check the consistency and flavor of the milkshake. If it's too thick to drink through a straw, blend in an additional 1 tablespoon of milk at a time until the shake has thinned slightly. If the flavor isn't as strong as you'd like, add more flavor syrup.
Pour the milkshake into a tall glass and enjoy!