In the bowl of a stand mixer, mix all of the dough ingredients, except for the butter, using the paddle attachment. Cover and leave for 20 minutes.
After 20 minutes, attach the dough hook and add the butter one pat at a time while kneading at a low speed. After all the butter has been added, knead until you reach the windowpane stage. This will take around 10 to 15 minutes.
Bulk Fermentation
Set the dough in a warm place and leave until it doubles, 2 to 4 hours, depending on the temperature of your kitchen. Alternatively, you may stick it in the oven with the light on.
Prepare the streusel
In a medium bowl, mix the flour, sugar and cinnamon. Add in the butter cubes and rub into the flour with your fingers until you have pea size pieces. Cover and refrigerate until needed.
Shape the streusel buns and final proof
When the dough has doubled, punch all the air out. Divide into 10 to 20 balls (roughly 55 g to 110g each). Then, flatten the balls until they are roughly 1.5 cm tall. Place on a lined baking sheet spaced around 4 cm apart. Cover with a damp tea towel and proof until doubled.
Once doubled, spray or brush the buns with water and divide the streusel topping evenly.
BAKE
Preheat the oven to 375°F/180°C.
Bake the buns for 18 – 20 minutes (depending on the size) until any exposed dough is golden brown. If you have a thermometer, you may check the internal temperature. It should be 90°C.
Cool and enjoy. Any leftovers can be frozen.
Notes
*To make the sweet starter/levain, mix 30g active starter, 30g water, 30g bread flour and 15g sugarUsing a fresh starter makes the dough less sour.