Add the dates, water, eggs, oil, baking soda, vanilla (optional), and a pinch of salt (optional) to a blender. Blitz until smooth.
Add the rolled oats and pulse just a few times - just enough to combine, only a few seconds!
Pour the mixture into a lined cake tin and sprinkle chopped walnuts on top.
Bake at 180°C for 45 minutes, until golden and set. Let it cool before slicing.
Once fully cooled, store the cake in an airtight container at room temperature for up to 2 days, or in the fridge for up to 3-4 days. For longer storage, slice and freeze the cake. Place parchment paper between slices and store in a freezer-safe container. Defrost at room temperature or warm gently in the oven or microwave.
