Place water and butter into the Thermomix bowl and melt 2 min/40°C/speed 1.
Add flours, yeast and salt and combine 6 sec/speed 6. Knead 2 min/Interval. If the dough is too sticky to easily work with, add a little more bakers flour.
Tip out onto a floured bread mat and roll into a tight ball. Place into a large bowl and cover with the bread mat. Rise until doubled and light and airy.
Preheat oven to 180°C and line 2 trays with baking paper.
Punch the dough down and form into 10 equal sized rolls. (approx 90g each) Place them onto prepared trays so they won't touch when risen. Cover with a cloth and allow to rise again for around 20 minutes.
Meanwhile bring a large pot of water to a boil. Add the baking soda to the water and give it a good stir to dissolve. Turn off the heat and cool slightly, but it should be quite warm for the next part.
Get ready, you need a good knife to cut your cross on top of each roll, the extra salt and the oven needs to be well heated. A slotted spoon is also helpful.
Slip 2 or 3 rolls into the water and turn a couple of times until the surface has sealed and is glossy. About 30 seconds per batch is plenty of time. Remove with the slotted spoon and return to the prepared trays.
Cut the cross on top of each bun as quickly as you can, sprinkle each with more salt flakes. Bake for about 15 minutes or until golden brown.
Serve with butter of your choice, unsalted is good with these are they have the lovely crispy flakes on top.