Preheat your oven to 175℃ (350℉). This is the perfect temperature to ensure your blondies bake evenly.
Line a 7x10-inch baking pan with baking or parchment paper. This step is crucial; it prevents the blondies from sticking and makes clean-up a breeze.
In a small nonstick saucepan, add brown sugar over medium-high heat. Stir with a wooden spoon until the sugar begins to melt and caramelize.
Once the sugar has melted, lower the heat and add in the butter, 1 tablespoon at a time. Keep stirring until the mixture is melted and smooth. This creates a luscious caramel sauce.
Remove the saucepan from heat, and slowly stir in the cooking cream and sea salt. Mix until the caramel is smooth and glossy, then set aside to cool slightly.
In a large mixing bowl, cream together the remaining butter and brown sugar until it becomes light and fluffy. This step is essential for achieving a tender blondie.
Beat in the eggs and vanilla, whisking until the mixture is smooth and well combined.
Gradually add in the plain flour and salt, mixing slowly until just combined. Be careful not to overmix; we want our blondies to be soft and chewy.
Gently fold in the cooled caramel sauce, swirling it through the batter. The caramel should be distributed evenly but not fully mixed in to create a marbled effect.
Pour half of the blondie batter into the prepared pan. Spread it evenly, then drop teaspoonfuls of Nutella onto the batter in a single layer. Use the back of a metal spoon or your fingertips to spread it gently.
Cover the Nutella layer with the remaining blondie batter, smoothing it out.
Bake for about 25 minutes, or until a toothpick inserted in the center comes out clean. The edges should be slightly golden, and the center still a bit soft.
Once baked, allow the blondies to cool for 10 minutes in the pan. Then, transfer them to a wire rack to cool completely. For the best experience, slice while still warm to enjoy that glorious Nutella ooze!