For the pancakes:
Melt the butter in a skillet over medium heat until it starts to brown. Add the rhubarb and sugar and cook for 5-10 minutes, until softened. Add a dash of water if the skillet starts to look dry. Alternately: For crisp, raw chunks of rhubarb, toss the rhubarb with sugar and 3 tablespoons of lemon juice and let sit while you make the batter.
Combine the milk with 1 tablespoon of acidic medium and let sit. Whisk together dry ingredients in a medium bowl: flour, sugar, baking soda, baking powder, and salt. Whisk the egg yolk into the "buttermilk" and gently stir into the dry ingredients until barely combined.
Stir in the egg white until a thick, lumpy batter forms--it's okay if the egg white is not completely incorporated. Stir in the melted, cooled butter and lemon zest, if using, until just combined. Lastly, fold in the rhubarb mixture.
For the crumble:
Whisk together the flour, sugar, salt and cinnamon, if using, then stir in melted butter or coconut oil. Flatten the mixture into a smooth layer along the bottom of your bowl.
Cook the pancakes:
Place a lightly greased skillet over medium heat. Use a quarter cup to dollop batter into the preheated pan, then press some crumble gently into the surface of each pancake. Cook until bubbles start to appear around the edges of each pancake, about 2-3 minutes. Flip pancake and cook for an additional 2 minutes or until cooked through. Repeat with remaining batter and crumble, lowering heat if the pancakes start to scorch; you shouldn't need to re-grease the pan. Serve immediately, topping pancakes with any extra crumble.