For the Cake:

1. Preheat the oven to 375°F. Grease and flour one 9-by-13-inch rectangular baking pan, or two 8-inch or 9-inch round cake pans.
2. In a large bowl, stir together coconut sugar and butter until creamy. Mix in the eggs until just incorporated, then mix in the kefir and water.
3. In a separate medium bowl, whisk together the einkorn flour, baking powder, and salt.
4. Stir the dry mixture into the prepared wet mixture just until combined. The mixture will be thick.
5. Gently fold in the blueberries, being careful not to overmix and break the berries.
6. Scoop the mixture into the prepared baking pans and bake for 30-40 minutes, until a tester inserted in the center comes out clean.
7. Transfer to a cooling rack, and let cool slightly for 15 minutes before removing from the pans to cool completely on the rack.

For the Topping:

1. Once the cakes have cooled completely, prepare the topping. Whisk together the heavy whipping cream and powdered sugar until stiff peaks form.
2. Decorate the cake with the whipped cream topping, and garnish with fresh blueberries. Serve.