Heat the oven to 175 C / 350 F. Line an 8-inch square baking pan with parchment paper, leaving an overhang on two opposite sides.

In a large mixing bowl, combine the granulated sugar and lemon zest. Rub them together with your fingertips for 30 seconds until the sugar feels damp and smells strongly of lemon.

Add the eggs, yogurt, oil, lemon juice, and vanilla to the sugar mixture. Whisk until smooth and combined.

Add the flour, baking powder, salt, and chia seeds. Fold with a rubber spatula just until no dry streaks remain. Do not overmix.

Let the batter rest in the bowl for 5 minutes so the chia seeds begin to hydrate.

Pour the batter into the prepared pan and spread it into an even layer.

Bake for 26 to 30 minutes, until a toothpick inserted in the center comes out clean and the top is pale golden.

Cool in the pan on a wire rack for 15 minutes, then lift out using the parchment overhang.

Whisk the sifted powdered sugar with lemon juice until smooth and pourable. It should coat a spoon but still drip off slowly.

Pour the glaze over the slightly warm cake and spread it to the edges. Let it set for 20 to 30 minutes at room temperature until matte before slicing.