In a bowl, whisk together flour, instant yeast, sugar, salt, and cornflour.
Add egg, lukewarm milk, oil, and vanilla extract. Mix until a rough dough forms.
Knead for 3–4 minutes so the dough starts coming together.
Add softened butter gradually, one cube at a time, kneading well after each addition.
Continue kneading until the dough is smooth, elastic.
Place it in a greased bowl, cover, and let it rise until doubled (about 1–1½ hours).
For the filling, melt chocolate and butter together until smooth.
Stir in cocoa powder, sugar, salt.
Add cream for a smooth, spreadable texture, then let it cool slightly.
Once the dough has risen, punch it down and roll into a rectangle about 10x12 inches.
Spread the chocolate filling evenly over the surface.
Sprinkle crushed pistachios on top of the filling.
Roll tightly from the long side to form a log.
Slice the log lengthwise with a sharp knife and twist both strands together, keeping the cut sides facing up.
Carefully place the twisted dough into a greased 9–10 inch round pan, tucking the ends underneath.
Cover and let rise again for 30–45 minutes until puffy.
Bake at 175°C (350°F) for 25–30 minutes, or until golden brown and fully baked through.
Syrup: combine sugar and water in a small saucepan, and cook until sugar dissolves completely. Brush it on freshly baked babka.
