In a stand mixer cream together the coconut oil or butter along with the brown sugar. Add eggs one at a time.
In a separate bowl, mix together the dry ingredients - flour, salt, baking powder and chia seeds. The chia seeds are totally optional.
Add in the dry ingredients alternating with buttermilk.
Fold in rhubarb.
Pour batter into a greased and parchment lined 9 inch springform pan (you can also use a 9 x 13 inch rectangle pan or a 9 inch round pan.
Mix together the additional sugar, nuts, cinnamon, and nutmeg. Sprinkle this mixture ontop of the batter.
Bake for 45 minutes at 350 degrees. Serve warm.