Butter an 8-inch (20cm) square cake pan and line with bottom with parchment paper. Preheat the oven to 375º (190ºC).
To make the white chocolate cake, in a bowl set over a pan of barely simmering water, warm the butter, white chocolate, lemon zest, and salt together, stirring gently, until the chocolate is completely melted. The mixture may not look smooth, which is normal.
In a medium bowl whisk together the sugar with the eggs, then whisk in the melted white chocolate mixture.
Use a flexible spatula to stir in the flour, mixing just until no visible bits of flour remain. Do not overmix. Scrape the batter into the prepared pan and bake for 30 to 35 minutes, until a toothpick inserted into the center comes out clean.
Remove the cake from the oven and as soon as it’s cool enough to handle, run a knife around the edge of the cake to release it from the pan, and set the cake on a wire cooling rack, removing the parchment paper. Let the cake cool completely.
Make the glaze by mixing together the powdered sugar with the lemon juice until smooth. Spoon the glaze over the cake, smoothing it over the top with a metal spatula or butter knife, letting some of it drip down the sides. Once the glaze has hardened, cut the cake into portions.
