Preheat the oven to 180°C and line a small baking dish or loaf tin with baking paper, leaving a little overhang to help lift the cake out after baking.
Peel the very ripe bananas, break them into chunks and place them in the blender jug so they are ready to blend smoothly.
Add the eggs, Greek yoghurt, soft curd cheese, vanilla extract and a pinch of salt to the blender, then blend until the mixture becomes completely smooth and creamy.
Taste the batter, add 2–3 tablespoons of agave syrup depending on how sweet your bananas are, and blend again briefly to incorporate the syrup evenly.
Sprinkle in the cocoa powder and pulse until it disappears into the batter and the mixture takes on a slightly deeper colour.
Let the batter rest for 3–5 minutes so it thickens slightly and prepares to bake more evenly.
Pour the rested batter into the prepared baking dish, tap the dish gently on the worktop to release air bubbles and smooth the surface with a spatula.
Bake at 180°C for about 15 minutes, or until the top looks set and lightly golden and the centre gives a gentle jiggle when you nudge the dish.
Remove the cake from the oven and allow it to cool for 10–15 minutes, then lift it out using the baking paper and slice into six portions.
Serve warm or chilled, plain or topped with Greek yoghurt, sliced banana, cocoa or nut butter, depending on how indulgent you want the cake to feel.