Preheat your oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan.
In a large bowl, combine the flour, sugar, baking powder, baking soda, and salt. Cut in the softened butter until the mixture resembles coarse crumbs.
In a separate bowl, beat the eggs, buttermilk, and vanilla extract together. Add the wet ingredients to the dry ingredients and stir until just combined.
Gently fold in the chopped rhubarb, making sure it is evenly distributed throughout the batter.
Pour the batter into the prepared baking pan and spread it out evenly.
In a small bowl, combine the flour and sugar for the crumb topping. Cut in the chilled butter using a fork or pastry cutter until the mixture resembles coarse crumbs. Sprinkle the crumb topping over the cake batter.
Bake in the preheated oven for about 40-45 minutes or until a toothpick inserted into the center comes out clean.
Remove the cake from the oven and let it cool in the pan for about 10 minutes. Then, transfer it to a wire rack to cool completely.
Once the cake has cooled, you can serve it as is or optionally dust it with powdered sugar. It pairs well with whipped cream or vanilla ice cream.