Warm about 1/2 cup of Whole Milk to lukewarm (not hot) and whisk in crumbled Fresh Yeast with a spoonful of Granulated Sugar; let sit for 5–10 minutes until bubbly to activate the yeast.
In a large bowl combine All-Purpose Flour with a pinch of Salt.
Create a well in the flour, add a beaten Egg, softened Unsalted Butter and the milk-yeast mixture, then mix until a soft dough forms.
Turn the dough onto a lightly floured surface and knead for 6–8 minutes until smooth and elastic; place in a lightly oiled bowl, cover, and let rise in a warm place until doubled in size (about 1–1½ hours).
While the dough rises, make the streusel by cutting cold Unsalted Butter into a mixture of All-Purpose Flour, Granulated Sugar and a small pinch of Salt until coarse crumbs form — you want a range of pea-to-fine-sized pieces for good texture.
Once the dough has risen, gently deflate it and press or roll it into a greased or parchment-lined baking pan to an even thickness; cover and allow to rest for 15–20 minutes so it puffs slightly.
Evenly sprinkle the prepared streusel over the surface of the dough, pressing lightly so the crumbs adhere but still remain loose and crumbly on top.
Bake in a preheated oven at 180°C (350°F) for about 25–35 minutes, or until the streusel is golden brown and the cake sounds hollow when tapped.
Allow the Streuselkuchen to cool in the pan for a short while so the streusel sets, then transfer to a wire rack to cool further; slice and serve slightly warm or at room temperature.
