Preheat oven to 350F. Line a baking sheet with parchment paper or coat lightly with oil.
In a medium bowl, mix together the flour, baking powder, salt , and sugar.
Cut the 4 tablespoons butter into the flour until it is pea-sized pieces or smaller. I use a fork for this. You can also use a pastry blender.
Add the non-dairy milk and mix until a cohesive dough forms. I find once it starts to come together that it is easier to use your hands.
Roll dough out on a floured surface to 1/4″ thick.
Spread brown sugar over dough, leaving a 1″ border at edge.
Dot the 2 tablespoons non-dairy butter on top of the brown sugar in small pieces.
Arrange the rhubarb on top making sure to leave 1″ around edges.
Wet all the edges lightly with a little bit of water (this is to help it stick together once it’s rolled).
Carefully roll the pastry up from the long side. Tuck in any filling that falls out. It doesn’t need to be super tight (that might tear the dough), a gentle close roll up is good. Press all the edges together. The center seam should be face up. Pinch the ends closed really well. The filling likes to leak out.
Arrange on baking sheet. Brush top with 1 tablespoon non-dairy milk.
Bake 40-45 minutes. The top will be light golden brown.
Let cool 15 minutes before slicing.
Slice and serve with ice cream, whipped topping, and/or yogurt.