Crumble the yeast into a small bowl. Add the water, ¼ teaspoon sugar and ¼ teaspoon salt. Cover the bowl with plastic wrap and leave it in a warm place until very frothy.
Melt the butter and margarine, but do not boil. Add the milk and cream, and set aside.
Sift the flour, sugar and salt together. Beat the eggs with the extra yolk until frothy. Add the flour mixture to the eggs, then add the yeast mixture. Beat with a dough hook for 10–12 minutes, or knead by hand. Place the dough in a bowl that has been sprayed with oil, and cover with plastic. Leave it in a warm place until the dough doubles in bulk, about 6 hours, depending on the weather.
Throw the dough onto a floured board, divide into three and knead each piece. Add as little flour as possible, as it will only make the dough heavy. Knead as little as possible, as it spoils the texture. The dough must be just manageable. Ensure there is a little flour on the rolling pin and roll each piece of dough out until about 20 cm (8 in) wide and 5 mm (¼ in) thick.
Make the cinnamon sugar by combining the sugar and cinnamon. Brush the dough generously with melted butter, sprinkle liberally with cinnamon sugar and sultanas, and roll up lengthwise. Cut the log into 5 cm (2 in) sections and place in the cups of large muffi n tins, sliced side up. Mix the beaten eggs with a little water and brush the top of the dough.
Mix the streusel ingredients and sprinkle generously on top. Leave to prove or increase in bulk for about 2 hours.
Preheat the oven to 180°C (350°F).
Bake for about 20 minutes. Boolkes freeze well. Reheat, covered in foil.