Mix ground flaxseed and water in a cup and leave to soak for 10 minutes.
Put all the dry ingredients (flour, ground pistachios, baking powder, coconut sugar) in a large bowl and mix them.
In another bowl, mix the wet ingredients (vanilla extract, agave syrup, plant-based drink, coconut oil and soaked flaxseed) and add them to the dry ingredients mixture and mix with a hand mixer until smooth.
Pour the mixture into two cake models with a diameter of 25 cm and bake for 25 minutes in a preheated oven at 180 °C (356 °F). (If you only have one model, the cake can be baked one after the other in the same model).
Put the cake on a cooling rack and let it cool. If the top of the cake is rounded, cut it off.