Make Rhubarb Compote
Combine 1 ¾ cups rhubarb, sugar, and water.
Boil over medium heat, then simmer 12-15 minutes until jammy.
Add remaining ½ cup rhubarb and simmer 2 minutes more.
Stir in food colouring, if using.
Cool completely.
Make Blondie Batter
Preheat oven to 350F.
Combine butter and brown sugar. Whisk until smooth.
Cool mixture if warm, then whisk in eggs.
Stir in vanilla.
Whisk together flour, baking powder, and salt.
Add flour mixture to batter and stir until just combined.
Stir in white chocolate, if using.
Bake and Finish
Spread batter into a 9x9-inch pan.
Dollop compote randomly on top.
Marble compote into batter with a skewer.
Bake 35-40 minutes, until set.
Cool completely in pan before cutting.
