Caramelize sugar in a saucepan over low heat until golden brown, stirring constantly.

Carefully add water to the caramel (it will splash).

Melt butter in the hot syrup and let it cool.

Preheat oven to 180 degrees Celsius.

In a mixing bowl, lightly beat eggs with a whisk.

Add condensed milk to the eggs and mix well.

Sift flour and baking soda into a separate bowl.

Pour the egg mixture into the flour mixture and mix well.

Add the cooled syrup to the batter and mix.

Pour the batter into a greased 8 x 3 cake pan.

Let the batter sit for 5 minutes to allow the baking soda to react.

Bake for 1 hour, or until a skewer inserted into the center comes out clean.

Let the cake cool in the pan for 5-10 minutes before turning it out.

Cool completely before cutting and serving.