Preheat the oven to 180°C (356°F) using top and bottom heat with fan assistance.
Place the almonds in a food processor or blender and grind them until they reach a flour-like consistency. Transfer to a mixing bowl.
Add the oats, xylitol, erythritol, and cocoa powder, then mix with a spoon.
Add the Greek yogurt and mix into a dough-like mass. (It will be slightly soft.)
Shape the mixture into a log and divide it into three equal parts. Roll each part into a ball.
Place the balls onto a baking tray lined with parchment paper, leaving space between them. Flatten each one with a spoon to form three equal rectangles, about 0.5 cm thick.
Bake the rectangles in the preheated oven for only 10 minutes. When you take them out, they may seem underbaked, but this is completely normal.
Let them cool for 10 minutes, then carefully peel off the parchment paper and place the rectangles onto a tray. Set aside.
Pour the pudding powder and xylitol into a bowl. Add a small amount of cold water, just enough to cover halfway, and stir until smooth and lump-free.
Bring the milk to a boil, then add the pudding mixture. Cook while stirring until you get a thick cream.
Spread part of the cream evenly over one of the baked rectangles.
Slice the apple into thin wedges and place one-third of the slices on top of the pudding layer.
Place the second rectangle on top, spread with pudding cream, and add more apple slices.
Add the third rectangle. Top with the remaining pudding cream and apple slices.
Refrigerate for 3 hours so the pastry layers can soften.
Before serving, dust the top with cocoa powder and cinnamon, then slice and enjoy.