Preheat oven to 350℉ (177℃). Line a 9x9-inch pan with parchment paper.
Cream brown sugar and butter for 2 minutes until light. Add egg and vanilla and mix well, scraping the sides and bottom of the bowl.
1/2 cup (113g) salted butter, 1 1/4 cups (250g) brown sugar, 1 large egg, 1 teaspoon pure vanilla extract
Add the flour, baking soda, and salt, mixing just until combined. Stir in the milk.
2 cups (282g) all-purpose flour , 1 teaspoon baking soda, 1/2 teaspoon salt, 1 cup whole milk
Toss the rhubarb with the remaining tablespoon of flour, then fold into the cake batter. Spread into the prepared pan.
2 cups chopped rhubarb
Combine the sugar and cinnamon, then sprinkle over the top of the cake. Bake at 350°F for 40-45 minutes, until a toothpick inserted into the center of the cake comes out clean.
1/4 cup granulated sugar, 1/2 teaspoon ground cinnamon