Cut the butter into cubes and leave at room temperature to soften
Grease the baking tray and medium bowl Fit the Dough Tool to the machine
Add sour cream into a small saucepan Heat over a low heat for about 3 minutes until warm Add butter into the saucepan Stir until melted and set aside
Add flour, yeast and salt into the Mixer bowl Attach the Mixer bowl to the machine and fit the splash guard Mix on speed Min for about 30 seconds until combined Transfer sour cream mixture into the Mixer bowl Mix on speed Min for about 1 minute until combined With the machine running, gradually add egg and egg white to the bowl Mix on speed Min for about 5 minutes until the mixture comes together and a dough is formed Let rest for 15 minutes Once the dough has rested, kneed on speed Min for about 5 minutes until smooth and elastic Transfer dough to the greased medium bowl Cover with oiled cling film Leave to prove in warm place for about 2 hours until doubled in size Transfer dough to a lightly floured worksurface and gently knead a few times Divide the dough into 3 equal pieces Roll each piece into a 45 cm (18”) log Braid the logs together, pinch and tuck each end underneath Transfer dough onto the baking tray Cover with oiled cling film Leave to prove in a warm place for about 50 minutes until risen
Preheat the oven to 190°C Add yolk and water to a clean small bowl Lightly whisk together to make the glaze Brush the dough with the egg glaze Sprinkle the dough with poppy seeds Bake for 35 minutes at 190°C until golden brown Remove from the oven and let cool for 10 minutes Turn out onto a cooling rack and let cool completely
Serve