Preheat the oven to 350 degrees and brush an 8 x 8-inch pan with the coconut oil.
Combine the flaxseed meal and water. Stir well and set aside for 5 minutes.
Place 1/2 cup of the chocolate chips in a small glass bowl and set over simmering water. Stir every minute or so until the chocolate is fully melted.
While the chocolate is melting, whisk together the coconut sugar, cacao powder, cassava flour, baking soda, and salt. Add the Greek yogurt, avocado oil, eggs, vanilla extract, flax mixture, and melted chocolate. Stir until smooth.
Add the remaining chocolate chips, and walnuts if using. Stir until well combined. Pour batter into the prepared pan.