Preheat the oven to 350°F. Grease a 9x13 pan and set aside.
Whisk flour, salt, & baking soda in a large bowl. Set aside.
In a small bowl, combine sugar, milk, melted butter, lemon juice, and vanilla. Stir in the egg.
Add the rhubarb to the flour mixture and gently mix.
Add the milk mixture to the flour mixture and stir with a spatula until combined.
Pour the cake mixture into the prepared pan.
Combine the sugar and cinnamon and sprinkle over the cake.
Bake 40-45 minutes, or until a toothpick comes out clean.
Cool and serve with ice cream.