Place a short tumbler or glass in the freezer to chill while you prepare the frappe — this keeps the drink colder longer and helps it stay frothy.
In a small bowl or measuring cup, combine the instant espresso powder, cocoa powder and sugar. Add about 2 tablespoons of the milk and whisk until the powders are fully dissolved into a smooth paste.
Pour the remaining milk and the vanilla extract into a blender. Add the dissolved espresso-cocoa paste and blend on low for 5–10 seconds to incorporate everything evenly.
Add the ice to the blender in two stages: start with half the ice and pulse 6–8 times, then add the rest and blend on medium-high until you reach a creamy, slushy consistency. Scrape down the sides if needed and pulse once more for uniform texture.
Remove the chilled glass from the freezer and pour the frappe into it. Top with a dollop of whipped cream if using and a light dusting of cocoa powder or a few coffee sprinkles.
Serve immediately with a straw and enjoy while icy and frothy.