Preheat your oven to 340°F (170°C).
Line a baking dish with parchment paper.
Mix the dairy-free milk and vinegar, then set aside to thicken.
Sift the flour, protein powder, cocoa powder, and baking powder into a large bowl.
In a separate bowl, combine the milk mixture, yogurt, oil, and Biscoff spread.
Pour the wet ingredients into the dry ingredients and gently mix until just combined.
Pour the batter into the prepared baking dish.
To make the sauce, pour the boiling water over the Biscoff spread, then mix in the brown sugar and cocoa powder until smooth.
Carefully pour the sauce mixture over the cake batter.
Bake for 25-30 minutes, or until the center is almost set.
Let the pudding cool slightly before serving.