Measure the flour into a big bowl and make a well in it.
Measure in the yeast and one spoonful of the sugar.
Pour over the water, flick some flour over the top of the water, and let it sit for one hour.
Add the rest of the dough ingredients EXCEPT the butter and knead well for 10 minutes. Then, add the butter and continue to knead for up to 20 minutes. Don't panic. The butter will melt and a lake of dough will form. Keep going! The dough will come together and darken in colour, getting golden streaks. It will be the texture of chewing gum. It's not alarming if you do it in a machine but it will take just as long.
Cover with plastic wrap and let it sit on the counter for 1-2 hours or until it has at least doubled in size.
While the dough is rising, make the filling ingredients by melting the butter and chocolate together. Stir in all the other filling ingredients except the pistachios. Cover with plastic and set aside.
When the dough has risen, scrape it out of the bowl onto a lightly floured surface. Divide it in two.
Using a rolling pin, roll the first piece into a rectangle that is about 1.5 times the length of your tin and about 1/2 cm thick.
If your filling has solidified, heat it VERY BRIEFLY just to soften it. You don't want it to be hot or it will kill the yeast. You just need it to be spreadable.
Spread half the filling over the dough, scatter over half of the pistachios, and roll it up along the long side. Wrap it in plastic wrap and place it in the freezer (seriously - this makes it easier to deal with later. Repeat with the other piece of dough. Leave them in the freezer for 10 minutes.
Generously butter your two bread tins.
After 10 minutes, remove the first sausage of dough the freezer. Unwrap it and, with a sharp knife, cut completely in half along its length. Lay the two halves next to each other, touching, with the cut sides facing up.
Twist two halves these together tightly, trying to keep the cut sides up the whole time - this is what makes the babka pretty. Gather it all up and gently snuggle it in one of the tins. Repeat with the other loaf.
Cover and let rest for about 1 hour. They will visibly rise to fill the tins completely
Preheat the oven to 375 F/190 C. Glaze the loaves with the egg wash and pop them in. Bake for 35 minutes but check after 20 to make sure they are not going too brown. If they are too brown, cover them with non stick baking parchment. When they are done, remove them from the oven.
If you are going to use the "after baking" optional extra ONE, melt together the butter and the sugar. Make sure the sugar is completely melted in so that it will not crystallise when it cools. Brush, and the pour this over the loaves while they are still in the tins. Let them cool in the tins and then remove.
If you are going to use the "after baking" optional extra TWO, measure the water and the sugar into a saucepan and gently heat until the sugar is completely melted. Brush, and the pour this over the loaves while they are still in the tins. Let them cool in the tins and then remove.
You can fill these with whatever you want, it does not need to be chocolate. At Easter it's nice to use marzipan with chopped almonds and poppy seeds. You can use butter/sugar/cinnamon or cardamom, or add cinnamon to the chocolate mixture, you can use whatever nuts you like - ground up or whole, you can add cranberries or raisins if that makes you happy, you can do jam if that's all you have or nutella if you are desperate. Anything that is delicious and beautiful that gives you joy.