Preheat your oven to 180°C (350°F) if using a conventional oven. If using a convection oven with a fan, refer to note 3 for temperature adjustments. Line an 8x8 inch baking pan with parchment paper, ensuring the edges are well-covered for easy lifting later.

In a medium-sized bowl, sift together the flour, cornstarch, baking powder, and salt. Mix thoroughly with a whisk or fork to ensure even distribution, then set the bowl aside.

Prepare the white chocolate by cutting it into small chunks if using a block, and roughly chop the walnuts or other preferred mix-ins. Keep them ready for later use.

In a separate large mixing bowl, combine the melted butter with the light brown sugar. Stir with a whisk until the mixture is well-blended but not overworked. If the butter is too hot, allow it to cool slightly before adding other ingredients to prevent cooking the eggs in the next step.

Add the eggs and vanilla essence to the sugar and butter mixture. Whisk gently until the mixture is smooth and cohesive. Avoid excessive whisking, as this could introduce too much air, altering the texture of the blondies.

Switch to a spatula and carefully fold the sifted dry ingredients into the wet mixture. Stir just enough to incorporate the flour mixture without overmixing, which can lead to dense blondies.

Gently fold in the prepared white chocolate chunks and chopped walnuts, ensuring they are evenly distributed throughout the batter.

Pour the batter into the prepared baking pan, smoothing the surface with the back of a spoon or an offset spatula. To eliminate large air pockets, gently tap the pan on the counter a few times.

Bake for 28-30 minutes, or until the top appears lightly golden and set. A toothpick inserted into the center should emerge with a few moist crumbs or slightly underbaked batter on the tip (see note 4 for details).

Allow the blondies to cool in the baking pan for approximately 45 minutes to firm up. Carefully lift them out using the parchment paper, transfer to a cutting board, slice into squares, and serve.