Soften the Dates: Place the chopped dates in a small heatproof bowl. Pour over enough hot water to cover them and let them stand for about 10 minutes until softened. Drain well and gently mash with a fork, leaving some small pieces intact.
Prepare the Toffee Sauce: Add the butter, dark brown sugar, heavy cream, and golden syrup to a medium saucepan. Cook over medium-low heat, stirring frequently, until the butter melts and the sugar dissolves.
Finish the Sauce: Bring the mixture to a gentle simmer and cook for 2–3 minutes until smooth and slightly thickened. Remove from the heat and stir in vanilla extract and salt. Set aside to cool slightly.
Prepare the Oven and Pan: Preheat the oven to 350°F (175°C). Line an 8-inch square baking pan with parchment paper, leaving some paper hanging over the sides for easy removal.
Make the Blondie Batter: In a large bowl, whisk together the melted butter, light brown sugar, and dark brown sugar until smooth. Add the eggs and vanilla extract and whisk until glossy and well combined.
Add the Dry Ingredients: In a separate bowl, whisk together the flour, cornstarch, baking powder, and salt. Gradually add the dry ingredients to the wet mixture and gently fold until just combined. Do not overmix.
Add Dates and Chocolate: Fold the softened dates and white chocolate chips into the batter until evenly distributed.
Fill the Pan: Transfer the batter to the prepared baking pan. Spread it evenly into the corners using a rubber spatula.
Add the Toffee Swirl: Drizzle a few spoonfuls of the prepared toffee sauce over the batter. Use a knife or skewer to gently swirl the sauce through the top.
Bake: Bake for approximately 28–32 minutes, or until the edges are golden and set while the center remains slightly soft. Avoid overbaking so the blondies stay chewy and moist.
Cool: Remove the pan from the oven and place it on a cooling rack. Allow the blondies to cool in the pan for about 20 minutes before lifting them out using the parchment paper.
Slice and Serve: Cut the blondies into squares. Warm the remaining toffee sauce gently and drizzle it over each serving. Finish with flaky sea salt if desired.