Preheat your oven to 350°F (175°C) and grease a 9x9 inch baking pan
In a saucepan, melt the unsalted butter over medium heat. Once melted, remove from heat and stir in the brown sugar until well combined
In a mixing bowl, beat the egg and vanilla extract together until smooth. Gradually add the melted butter and sugar mixture, whisking until fully incorporated
Stir in the lemon zest, orange zest, lemon juice, and orange juice until evenly distributed
In a separate bowl, whisk together the all-purpose flour, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined
Pour the batter into the prepared baking pan and spread it out evenly with a spatula
Drizzle the caramel sauce over the top of the batter, then use a toothpick or knife to swirl it into the batter for a marbled effect
Bake in the preheated oven for 25-30 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out clean
Remove from the oven and let the blondies cool in the pan for at least 15 minutes before slicing and serving