Giant Lemon Raspberry Brioche Bun

A giant lemon raspberry brioche bun with a lemon sugar butter & raspberry filling and a lemon raspberry glaze.

In a small saucepan over medium heat, continuously stir together the flour and milk until the mixture and thickens into a thick slurry. Take off the heat and set aside. Alternatively, whisk together the flour and milk in a microwave safe bowl. Microwave in 30 second intervals, stirring after each, until the mixture becomes a thick slurry.
Add the flour, sugar, yeast, salt, butter, egg, milk, and tangzhong mixture to the bowl of a stand mixer fitted with the dough hook attachment.
Knead the mixture on medium speed for ~15-20 minutes. The dough is ready when it is no longer sticking to the sides or bottom of the bowl, and passes the windowpane test.*
Once the dough is done kneading, shape the dough into a ball and place it back into the mixing bowl. Cover the mixing bowl with plastic wrap, and place it in a warm area to rise until doubled in size (~1 hr.)**

Generously butter an 8" cake pan. Cover the bottom with parchment paper and set aside.
Add the sugar to a small mixing bowl. Zest two lemons into the sugar, then use your hands to rub the zest into the sugar until combined.
Add in the softened butter, and use a fork or spatula to mix the butter with the sugar until the mixture is cohesive.
Once the dough has finished rising, lightly punch it down to remove any excess air in it. Transfer the dough to a clean and lightly floured surface, then roll it out to a 12″ x 16″ (30 x 40 cm) rectangle.
Place dollops of the lemon-sugar-butter filling over the dough, then gently spread it out into an even layer over the dough.
Slice your raspberries in half and evenly scatter them over the dough.***
From the longer side, roll the dough up into a tight cylinder. Use a knife to cut the dough in half vertically, leaving ~1" connected at the top.
Twist the two pieces of dough over themselves, then roll it into a tight knot. Tuck the end of the dough in under the big roll.
Place the roll into your prepared springform pan. Lightly cover the pan with a towel, and let the roll rise in a warm environment until doubled in size (~30 minutes).

Preheat the oven to 350°F / 175°C.
Make an egg wash by whisking together an egg with 30g of milk, then brush it over the roll right before baking. (You will not use all of the egg wash, store the rest in the fridge or freezer for other pastries!)
Bake for 25-30 minutes, until the top of the cake is golden brown and the internal temperature of the cake has reached 180°F / 82°C.

In a small bowl, mash the raspberries, then mix in the lemon juice.***
Add the powdered sugar to a small mixing bowl. Add in 1 tbsp. at a time of the lemon-raspberry mixture into the powdered sugar, mixing after each, until you reach your desired glaze consistency.
Allow the lemon raspberry roll to cool to room temperature, then spread the glaze over the top of it. Slice, serve, and enjoy!