Preheat the oven to 180°C (350°F). Line a cake tin with parchment paper.
Add the dates, water, eggs, olive oil, maple syrup, and baking soda to a blender. If using, add the vanilla and salt. Blend until completely smooth.
Add the rolled oats and pulse only a few times, just enough to combine. Do not overblend; a few seconds is all it takes.
Pour the mixture into the prepared cake tin and sprinkle the chopped walnuts evenly over the top.
Bake for 45 minutes, or until golden and set in the center. A toothpick inserted should come out clean.
Allow the cake to cool completely before slicing.
Store the fully cooled cake in an airtight container at room temperature for up to 2 days, or refrigerate for up to 3 to 4 days.
