Melt the butter and set it aside to cool slightly.
In a large bowl, beat the eggs and sugar for 3–4 minutes, until pale, thick, and slightly increased in volume.
Mix in the vanilla extract and lemon zest.
In a separate bowl, whisk together the flour, baking powder, and salt.
Gently fold the dry ingredients into the egg mixture in two additions, mixing only until no visible streaks of flour remain.
Slowly pour in the melted butter and fold gently until the batter is smooth and evenly combined.
Cover the bowl and refrigerate the batter for 1 hour.
Preheat the oven to 190°C (375°F).
Generously grease the madeleine pan with softened butter. Dust the molds lightly with flour, then tap out any excess. If using a silicone mold, lightly grease only if necessary.
Spoon the chilled batter into the molds, filling each about three-quarters full. Gently press 2–3 small blueberries into the batter in each mold. Do not spread the batter flat.
Bake for 10–12 minutes, until the madeleines are lightly golden around the edges and spring back when gently pressed.
Let them rest in the pan for 2–3 minutes, then carefully remove them and transfer them to a wire rack.
Allow to cool slightly, then dust with powdered sugar before serving.