Make the cake

• Generously butter a 9 x 13 inch flat baking dish. Preheat the oven to 350°F
• In the bowl of the stand mixer [or large mixing bowl if using a hand mixer], beat the butter and brown sugar until very fluffy and light.
• Beat in the eggs, again until very fluffy and light.
• Beat in the vanilla
• In another, smaller bowl, whisk together the dry ingredients - flour, salt and baking soda.
• Add the flour mixture in thirds, alternating with the buttermilk. [1/3 of the flour, 1/2 of the milk, 1/3 of the flour, 1/2 of the milk, 1/3 of the flour] Beat briefly after each addition. You will have a fairly smooth batter.
• Remove the mixing bowl from the stand, and using a rubber spatula, gently mix in the rhubarb.
• Scrape the batter evenly into the buttered baking dish.
• You can use the same bowl and beaters as the cake batter- no need to clean them. beat together the topping ingredients, scraping the side of the bowl as needed. You want a uniform, crumbly mixture.
• Evenly distribute dabs of the topping over the top of the batter.
• Bake for 40 to 45 minutes, turning once to ensure even baking. Test the cake for doneness, by inserting a toothpick near the center. Try to avoid the pieces of rhubarb - there should be no more than a moist crumb or two on the toothpick. Another 5 minutes of baking time is a good idea if you aren't certain.
• Cool the cake on a rack. I usually just serve right from the baking dish. This really doesn't need anything at all on it, but it is good with vanilla ice cream, whipped cream, a dollop of Creme Fraiche, or just a dusting of powdered sugar.