Preheat oven to 350 degrees and line a 9 by 13 inch baking pan with parchment paper

In a large bowl combine flour, baking soda and salt. Set aside.

In the bowl of electric mixer cream together brown sugar and butter until fluffy. (About 2 to 3 minutes).

Add egg and vanilla and beat on medium speed until combined.

Add 1/2 the flour and beat on low just until incorporated. Add half the buttermilk and beat on low just until incorporated. Repeat with remaining flour and buttermilk. DO NOT OVER MIX.

Remove bowl from mixer and fold in 1 cup of the chopped rhubarb. Spread into prepared pan.

Combine all of the crumble ingredients in a large bowl, using clean hands to rub together to create a crumbly mixture. Spread evenly over cake batter, pressing in gently. Sprinkle remaining 1/2 cup rhubarb over the crumble.

Bake for 40 to 45 minutes until toothpick comes out with just a few crumbs. The edges of the cake will be nicely browned.

Transfer pan to a wire rack to cool completely. Remove from pan and cut into generous squares.