Preheat the oven to 350 degrees F. Prepare an 8" x 8" square baking pan by lightly oiling it or lining it with parchment paper.
In a stand mixer or a large mixing bowl, cream together the butter and coconut sugar until fluffy.
Add the eggs and vanilla extract and mix to combine.
In a separate bowl, sift together the flour, baking powder, baking soda and salt.
Combine the dry ingredients with the wet ingredients and mix just until incorporated.
Mix in the blueberry yogurt and then the blueberries.
In a small bowl, mix together the brown sugar, walnuts and cinnamon.
Pour half of the cake batter into the prepared baking pan.
Sprinkle half of the brown sugar/nut mixture on top of the batter.
Pour the remaining batter on top and use a rubber spatula to smooth it out evenly.
Sprinkle on the remaining brown sugar/walnuts.
Bake for 30 to 40 minutes until the cake tests clean. To test for doneness, insert a digital thermometer into the center of the cake. It is ready if the internal temperature is between 190 and 205 degrees Fahrenheit.