Blueberry Brioche Crumb Cake

Light and fluffy brioche topped with a generous layer of homemade wild blueberry jam and lemon streusel.

In a microwave safe measuring cup, heat the milk to 100°F / 38°C. Whisk in the yeast and the sugar. Set aside until the mixture begins to foam. (~5 minutes)
While waiting, add the flour, salt, egg, and butter to the bowl of a stand mixer fitted with the dough hook attachment. When the yeast mixture begins to foam, add it in to the rest of the ingredients.
Knead the dough on low speed for 1 minute to allow the ingredients to come together. Increase the speed to medium-high and knead for an additional 15-20 minutes, or until the dough passes the windowpane test. (This will happen after the dough has cleaned the sides and bottom of the bowl)
At this point, you could cover the dough and put it in the fridge to rise overnight, or let it rise in a warm environment until it's doubled in size, approximately 1 hour.
While the dough is rising, start on the blueberry jam and lemon streusel.

Add the wild blueberries, sugar, and lemon zest to a medium saucepan set over medium heat. Stir occasionally until the mixture comes to a simmer, then let it simmer for 5 minutes.
Make the corn starch slurry by whisking together the lemon juice and corn starch in a small bowl. Then, while mixing the blueberries constantly, stream in the corn starch slurry. Continue mixing constantly until the jam thickens, then remove it from the heat and set it aside. (If letting the dough rise overnight, cover the jam and place it in the fridge until you're ready to use it.)

In a medium sized bowl, melt the butter. To the butter, add the sugar, flour, and lemon zest. Use a fork to mix the ingredients together until you have formed clumps of dough. Set aside. (If letting the dough rise overnight, cover the streusel and let it sit at room temperature until you're ready to use it.)

Line your cake pan! This is not a cake that you want to flip upside down to get it out of the pan. Butter an 8" or 9" cake pan, and get out a sheet of parchment paper. Crinkle up the parchment paper, then un-crinkle it and place it in your buttered pan, pressing the parchment up the sides of the pan. This will give you a way to lift the cake out of the pan super easily later on.
Once the dough has doubled in size (or after taking it out of the fridge after rising overnight), lightly punch it down to remove excess air.
Form the dough into a ball, then place it in a greased and parchment lined 8" baking dish. Use your hands to push the dough down into the pan so that the dough is an 8" circle on the bottom of the pan. If the dough keeps snapping back, leave it to rest for 5 minutes before trying again. Cover the dough with plastic wrap or a towel and leave to rise at room temperature until doubled in size (~30 min.)
Preheat the oven to 350°F / 175°C.
Once the dough has doubled in size, spread the wild blueberry jam into an even layer over it. Then, sprinkle the streusel in an even layer over the jam.
Bake the cake for 30-35 minutes, until the internal temperature is 190°F / 88°C.
If the dough happens to rise up on the sides more than in the center, you can gently push it down with a spatula once it comes out of the oven. Wait for the cake to cool to room temperature, then slice it, serve, and enjoy!