In a small mixing bowl, dissolve yeast in warm milk. Let stand until creamy, about 10 minutes.
Prepare a sweet dough by creaming 1/4 cup sugar with the margarine and 1/4 teaspoon salt. Add 1 egg and beat with electric mixer one minute until well blended. Add 2 1/2 cups flour, the yeast mixture and 1 tablespoon vanilla. Mix three minutes with dough hook or with hands. Turn dough out on floured board and knead for one minute. Place in a lightly greased bowl, cover with a towel and set in a warm place to rise for one hour.
While dough is rising, prepare the topping: In a large bowl, combine 2 1/2 cups sugar, 250g butter and a pinch of salt. Add 1 egg and the golden syrup. Mix enough to incorporate. Add 2 1/4 cups flour, 1/4 cup (65ml) water and 1 teaspoon vanilla. Mix together well and set aside.
Divide dough into two pieces. Place each into a greased 23cm round cake tin. Crimp edges halfway up sides of pans to keep the gooey butter topping from running out underneath. After dough is spread out, punch holes in dough with fork to keep it from bubbling when baking.
Divide topping into two equal parts. Spread over dough in each pan. Let cake stand for 20 minutes.
Preheat oven to 190 degrees C. Bake cakes in preheated oven 30 minutes. Do not overbake; the topping should stay gooey. After cakes are cool, sprinkle with icing sugar.