Preheat the oven to 350 degrees. Make the crumb topping: Combine the brown sugar, salt and cinnamon together in a small bowl. Pour in the melted butter and stir until combined. Add the flour and stir until a thick cohesive dough forms that holds together when you squeeze it in your hand. Set aside.
Make the cake: Butter and flour a 9 inch square baking pan. Whisk the flour, baking powder, baking soda and salt together in a medium bowl and set aside.
Place the butter and brown sugar in the bowl of a stand mixer. Mix on medium speed using the paddle attachment until fluffy, about 2-3 minutes. Add the egg and vanilla and mix until fully combined.
With the mixer on low, add one-third of the buttermilk followed by one-third of the dry ingredients. Repeat two more times, finishing with the dry ingredients. Mix until the batter just comes together. Gently fold in the rhubarb with a rubber spatula.
Transfer the cake batter to the prepared pan and sprinkle the crumb mixture over the top by squeezing the mixture in your hand and scattering it evenly over the cake in "crumbs" that are no larger that ½ inch in diameter. Bake for 40-50 minutes or until the crumb topping is golden brown and a tester inserted in the center of the cake comes out clean. Cool to room temperature before slicing and serving.