1 1/4 cups milk
1 tablespoon white distilled or cider vinegar
2 1/4 cups unbleached all purpose flour
1 teaspoon baking soda
1/2 teaspoon fine sea salt
8 tablespoons (1 stick) unsalted butter, at room temperature
1 1/4 cups packed light or dark brown sugar
1 large egg
3 cups rhubarb sliced 1/2 inch thick

Topping
1/2 cup packed light or dark brown sugar
1/2 cup old fashioned rolled oat
1 1/2 teaspoon ground cinnamon

Position a rack in the center of the oven and preheat at 350*F. Lightly butter and flour a 9 x 13 inch baking pan, tapping out the excess flour. Combine the milk and vinegar and let stand until the milk curdles, about 5 minutes. Mix the flour, baking soda and salt to combine. Cream the butter and brown sugar together in a medium bowl with an electric mixer on high speed until light and fluffy, about 3 minutes.

Beat in the egg. Beginning with the flour, in three additions, use a spoon to stir in alternating portions of the flour and curdled milk, mixing just until smooth. You can use the mixer on low speed, but do not over mix. Fold in the rhubarb.
Spread evenly in the pan. Mix the brown sugar, oats, and cinnamon in a small bowl with your fingers until blended. Sprinkle evenly over the batter. Bake until a toothpick inserted in the center of the cake comes out clean, 35 to 40 minutes. Cool in the pan on a wire rack.