Preheat your oven to 325°F (165°C) and prepare a 9×9 inch metal baking pan by lightly greasing it and lining it with parchment paper.
In a double boiler over medium-low heat, melt the butter and white chocolate together, stirring continuously until smooth. Then, remove from heat.
In a large bowl, whisk together the eggs, egg yolks, granulated sugar, lemon zest, and vanilla until pale and smooth.
Pour the melted butter and white chocolate mixture into the egg mixture, whisking until well combined.
Gently fold in the flour and salt until just combined.
Spread the lemon brownie batter evenly into the prepared pan and smooth the top with a mini offset spatula.
Bake for 32-36 minutes, until the edges are set and the center is slightly gooey. If the brownies are browning too quickly, loosely cover the pan with parchment paper towards the end of baking.
Allow the brownies to cool completely in the pan on a wire rack before cutting into squares.