Whisk the base. In a medium bowl or jar, whisk together the milk, Greek yogurt, protein powder, cocoa powder, sweetener, vanilla, and salt until smooth.
Break up any protein or cocoa clumps before adding the seeds. A small whisk works best.
Add nut butter. Stir in the nut butter until fully incorporated. It should look glossy and thick, like a thin brownie batter.
Stir in chia seeds. Add the chia seeds and mix well, scraping the bottom and sides so no seeds clump together.
Let the mixture sit for 3–5 minutes, then stir again to evenly distribute the seeds.
Chill to thicken. Cover and refrigerate for at least 1–2 hours, or overnight for the thickest texture. The chia seeds will swell and create a pudding-like consistency.
Adjust texture. After chilling, give it a good stir. If it’s too thick, add a splash of milk.
If it’s too thin, stir in another teaspoon of chia seeds and chill 10–15 more minutes.
Taste and top. Taste for sweetness and salt. Add a pinch more salt to boost chocolate flavor, or a drizzle of sweetener if needed. Top with chocolate chips, berries, or nuts just before serving.