In the medium bowl, whisk together the whole wheat flour, all-purpose flour, baking soda, and salt. In the bowl of the stand mixer, cream together the butter and sugar until the mixture is light and fluffy. Beat in the egg and scrape down the sides of the bowl. Stir the vanilla extract in with the buttermilk. Add ⅓ of the dry ingredients and mix, then half the buttermilk, and mix. Repeat, and finish with the last of the dry ingredients. Fold the rhubarb and strawberries into the batter,
Spread the batter evenly among the 2 pans and sprinkle on the topping. Bake in the preheated oven until the top is golden brown and a tester comes out clean, about 25 to 30 minutes (30 to 35 minutes for a 9×13 cake.) Allow to cool on a wire rack for at least 20 minutes. Serve with ice cream or whipped cream.