Preheat the oven to 350 degrees F. Line a baking tray with parchment paper.
Place the rhubarb on the tray in a single layer and scatter over the sugar. Scrape out the vanilla seeds over the rhubarb and throw on the pod along with the orange juice and zest Cover tightly with foil. Bake for 12 minutes, until the rhubarb is just soft. Set aside to cool. Drain the rhubarb from its syrup and set aside, reserving the syrup for another dish.
Line a rectangular baking pan, approximately 10 x 8 inch.
Melt the butter over medium heat until melted and light golden brown.
In a large mixing bowl, whisk the browned butter and sugar until well combined and not lumpy.
Add the egg and vanilla extract and stir until smooth.
Place a sifter over the bowl, add flour, baking powder, and salt. Sift the dry mixture over the wet mixture. Whisk everything until just combined.
Fold in the chopped white chocolate and sliced almonds, reserving 2 tablespoons of almonds.
Pour the batter into the prepared pan and use an offset spatula to smooth into an even layer. Top with the roasted rhubarb. Scatter the rest of the almonds over the top.
Bake until the edges are browned and pulling away from the pan, and a tester inserted in the center comes out clean, 30 to 35 minutes.
Cool completely in the pan on a wire rack. Grab the excess parchment and lift the blondie slab out of the pan and onto a cutting board. Cut into 12 squares.
Sprinkle with powdered sugar before serving. (optional)