Melt the butter (don't let it boil) and set aside to cool slightly. In the meantime, line an 8x4 inch baking dish with baking paper (see notes for pan options). Preheat the oven to 175 C.
In a mixing bowl, beat the eggs and sugar with a hand mixer on medium speed, until pale and slightly thickened, 2 to 3 minutes. Stir in the vanilla, lemon juice and zest.
Now sift in the flour and salt, and beat on low speed briefly, until combined.
Add the melted butter and stir to combine into a smooth, runny batter.
Pour into the prepared baking dish and bake for 25 to 30 minutes, until the top has browned, and a toothpick poked in the center comes out with a few moist crumbs. This is the key to the gooey and dense texture. If you bake the blondies until they're completely dry, the texture will be more cakey. If you prefer that, feel free!
Let the blondies cool completely to room temperature. They will sink a little and firm up as they cool. When ready to glaze, simply combine the sugar and lemon juice, adding the juice slowly to make a runny glaze. You may need less or more of the juice. (See the recipe notes for more about the glaze)
Spread the glaze all over the blondies and let it set for about 15 minutes. Then slice and dive in!
The blondies are quite soft and gooey, so while they can be stored at room temperature for 2 to 3 days, in very warm weather I prefer to store them in the fridge where they will keep for a week or so. I liked them at room temperature as well as cold from the fridge when they are extra fudgy. Happy baking!