Prep the pan. Line a rimmed baking sheet with parchment paper. For thicker bark, use a smaller tray; for thinner bark, use a larger one.
Mix the base. In a bowl, whisk Greek yogurt, protein powder, sweetener, vanilla, and a pinch of salt until smooth.
Taste and adjust sweetness—cold dulls sweetness, so make it slightly sweeter than you think.
Spread it out. Pour the mixture onto the lined tray and spread to about 1/4-inch thickness with a spatula. Keep edges even for cleaner pieces.
Add toppings. Scatter berries and nuts or seeds evenly. Press lightly so they set into the yogurt.
Chocolate touch. Melt dark chocolate in short microwave bursts, then drizzle over the top.
Add nut butter in small spoonfuls and swirl with a toothpick if using.
Freeze. Place flat in the freezer for 2–3 hours, or until completely firm.
Break and serve. Lift the parchment out, break the slab into pieces, and enjoy immediately. For clean, even squares, score with a knife after 45–60 minutes of freezing, then finish freezing.