Preheat the oven to 375 (F). Prepare an 8-inch square baking pan by lining with parchment paper, leaving a 2-inch overhang on two sides. Spray the parchment paper liberally with cooking spray.
In a medium, heavy bottomed pan over medium-low heat, melt 1/2 cup unsalted butter until it starts to brown, foam slightly and emit a nutty aroma. Set aside and allow to cool slightly on a wire rack.
After around 5 minutes, whisk together the butter, 1/2 cup packed dark brown sugar and 1/3 cup granulated sugar until smooth. Whisk in 1 large egg and 1 teaspoon pure vanilla extract until fully incorporated.
Sprinkle 1 cup all-purpose flour, 2 teaspoons ground cinnamon, and 1 teaspoon kosher salt over the liquid mixture (from the 3rd step). Use a rubber spatula to fold in the dry ingredients until just combined — at this point, it’s okay to undermix and have one flour streak or two. Fold in 1 cup white chocolate chips.
Transfer the blondie batter to the prepared baking pan, using an offset spatula to create a smooth top. Spread the roasted rhubarb on top of the blondie batter evenly until most of the batter is covered, before scattering the leftover 1/2 cup white chocolate chips over the rhubarb.
Transfer to the preheated oven and bake for 40 to 45 minutes, or until the edges are golden brown. If the rhubarb and white chocolate looks like it’s browning too fast, create an aluminum foil tent over the pan. DO NOT OVERCOOK, or you’ll get tough, chewy blondies and I’ll cry. It’s okay for the center of the blondies to look a little bit wet as long as the edges are set and golden brown. Once the blondies are finished, transfer to a wire rack and cool completely.