Preheat the oven to 330°F (165°C) on conventional (non-fan) mode. Line a 9x5-inch/23x13 cm loaf pan with parchment paper, allowing an overhang for easy removal.
In a medium saucepan over low, gentle heat, melt the chocolate and butter together, stirring until smooth and glossy. Remove from heat and whisk in the sugar until combined. Let the mixture cool for 2 to 3 minutes before adding the eggs to prevent scrambling.
Add the eggs one at a time, whisking vigorously after each addition until fully incorporated and the batter is smooth.
Fold in the cornstarch and fine salt until just combined; do not overmix.
Pour half the batter into the prepared pan, spreading it evenly. Place the blondie chocolate bar in the center on top. Dollop the Nutella over the bar in spoonfuls, then cover with the remaining batter, smoothing the surface.
Sprinkle flaky sea salt over the top. Bake for 25 to 30 minutes, until the edges are set and the top forms a thin crust but the center still jiggles slightly when the pan is gently shaken.
Cool the brownies in the pan on a wire rack for at least 30 minutes to set before slicing with a sharp knife. Serve at room temperature.