Into a pot, mix water roux ingredients, stir to combine and try to make it as smooth as you can. Heat on low heat while keep stirring until thickened. Once you see that it's starting to boil, turn it off and beat fast with wire whisk. Let cool to room temperature
Once cooled, add in sugar, yeast, yolks, and water, stir to combine well with the water roux. Lastly add in salt, and stir again.
Add in flour a little at a time and knead. During the kneading, add in margarine and knead until elastic and smooth (about 10-15 minutes). Let rest for 1.5 to 2 hours long, it will be doubled in size. Punch down dough and knead again a little and divide into 18 portions (about 45-50 g each, depending on the weight, you can make between 15-20 portions), let rest for 10 mins.
Once rested, flatten each portion and place 1 butter cube in center and pinch dough to seal, creating round bun shaped. Place buns on lightly greased pan and let rest for 30-40 minutes until doubling in size again, covered.
Meanwhile, make the coffee topping!
Beat all until combined well then spoon into piping bag/ziplock bag and snip the corner
Preheat oven to 200 C. Pipe topping on buns and bake for 12 minutes.