Preheat oven to 325°F. Lightly grease an 8x8-inch baking dish and set aside.
Separate egg whites and yolks. Beat egg whites until stiff peaks form and set aside.
In a separate bowl, beat egg yolks with sugar until pale and creamy. Mix in melted butter and water.
Add flour, cocoa powder, and salt, then mix until fully incorporated.
Slowly add in milk and vanilla, mixing until smooth. Gently fold in beaten egg whites. The batter will be thin and slightly lumpy.
Pour the batter into the prepared pan and bake for 50–60 minutes, or until the center is just set and the top is slightly firm.
Cool completely. Chill in the fridge for at least 2 hours before serving.
Dust with powdered sugar before slicing.