Preheat the oven to 350 degrees F.
For the cake, combine the all-purpose flour, whole wheat flour, salt, baking powder and ground ginger. Whisk together.
Place the brown sugar, butter, and vanilla In the bowl of a stand the mixer and beat on medium high until fluffy, 3-4 minutes. Scrape the bottom and sides of the bowl.
Add the eggs, one at a time, beating after each addition until well combined. Add the vanilla and mix. Scrape down the sides and bottom of the bowl.
Add 1/3 of the flour mixture and beat on low speed until flour is just mixed in. Do not over mix. Add half of the buttermilk, beat until combined. Add half of the remaining flour and mix on low until just combined. Add the remaining buttermilk, mix on low and then add the remaining flour and mix on low until combined. Scrape the bottom and the sides of the bowl and mix on low speed for another 10-20 seconds to make sure everything is well combined.
Place the batter in the pan and use an offset spatula to spread the batter evenly around the pan.
Evenly spread the crumb and chopped rhubarb over the top of the batter by first spreading half of the rhubarb over the top of the batter, then spread all of the crumb mixture, then the remaining rhubarb.
Place pan in the oven and bake for 40-45 minutes. The temperature of the cake in the center should be about 202 degrees F. Remove from the oven and cool in the pan for 30 minutes. Use the parchment sling to remove the cake from the pan.
Allow the cake to cool completely before cutting and serving.