Heat the milk in a small saucepan until just simmering.
Add the milk to a large mixing bowl, then whisk in the sugar and butter until melted.
Check the temperature with the tip of your finger. The mixture should be just warm to the touch. Add the yeast and stir to combine.
Set aside for the yeast to bloom for 15 minutes. It should be visibly expanded - if not, see tips below. Once the yeast has bloomed, whisk in the egg and egg yolk.
Add the flour, cinnamon, and salt to the bowl. Mix with a wooden spoon or spatula until a shaggy dough forms, then turn it out onto a lightly floured surface and knead until a soft, smooth dough forms.
Return the dough to the mixing bowl and cover with a tea towel. Set aside to rise until doubled in size, about an hour.
Once the dough has risen, line a large baking sheet with parchment paper and preheat the oven to 350°F (180°C).
Make the streusel. Add the flour, sugar, softened butter, and salt to a mixing bowl. Use your hands to blend into a crumbly mixture, then set aside.
Roll the dough out to a large rectangle (about 30x40cm or 12x16 in.). Place it onto the prepared baking sheet. Top in an even layer with the prepared apples.
Top with an even layer of the streusel.
Bake for 30-35 minutes, or until the edges are a dark golden and the top is a pale gold. Cool for 15 minutes on the tray before removing and slicing.
Leftovers keep for a day or two in a sealed container, but the streusel will soften. This cake can be frozen, but it's best fresh.