Preheat oven to 350. Line a baking tray with parchment paper.
Place rhubarb, sugar and vanilla seeds on the tray and combine well. Bake for about 20 minutes until sugar has turned bubbly and syrupy and rhubarb has softened. Set aside to cool.
In a large mixing bowl, whisk butter and sugar until well-combined. Add egg and vanilla extract, whisk until smooth. Sift dry ingredients into the wet mixture. Stir until combined, stir in rhubarb. Once lightly combined, fold in walnuts and white chocolate. Place a new layer of parchment over baking tray and spoon scoops of dough onto tray, about one ice cream scoop full. Bake at 350 for 15-20 minutes until edges are browned. Take out and let cool before moving to a cooling rack.