Prepare the pan: Lightly grease a 9-inch pie dish or springform pan. Line the bottom with parchment if using a springform.
Make the crust: In a food processor, pulse oats and nuts until they form a coarse meal. Add cocoa powder, salt, maple syrup, and coconut oil.
Pulse until the mixture clumps when pressed. If dry, add 1–2 teaspoons more oil.
Press and set: Firmly press the crust mixture into the pan, working up the sides. Use the bottom of a measuring cup for a smooth, compact surface.
Chill in the fridge while you make the filling.
Melt the chocolate: Place chopped dark chocolate in a heat-safe bowl. Warm the coconut milk until steaming (not boiling), then pour it over the chocolate. Let sit 2 minutes, then whisk until smooth and glossy.
Build the filling: Whisk in maple syrup, vanilla, salt, and cocoa powder if using.
Let the mixture cool to just warm, then fold in Greek yogurt until fully smooth. Do not add yogurt to hot chocolate or it may curdle.
Fill the crust: Pour the filling into the chilled crust. Smooth the top with a spatula. Tap the pan gently on the counter to release air bubbles.
Chill to set: Refrigerate for at least 4 hours, preferably overnight, until firm and sliceable.
Garnish and serve: Top with berries, shaved chocolate, or toasted coconut.
Slice with a warm knife for clean edges.