Blueberry Filling

1. Add all of the ingredients to a saucepan over medium heat and mix to combine.
2. Continue cooking until the mixture starts to break down and bubble. Stir occasionally to prevent the bottom from burning. Heat the mixture for about 30 minutes or until reduced by half. It should have thickened, testing to see if the mixture coats the back of a spoon.
3. Scoop the jam into a heat safe dish. Cover and chill. 

Brioche Dough

1. Whisk together the flour, sugar, yeast, and zest in the bottom of the bowl of a stand mixer.
2. With the dough hook attachment, mix in eggs, milk and vanilla until it comes together into a somewhat dry clump.
3. With the mixer on low, add the salt then the butter, one tablespoon at a time, until it’s fully incorporated into the dough. Then, run the mixer on medium speed for about 10 minutes until the dough is completely smooth. Stop and scrape down the bowl a couple times during this process. You want the dough to pull away from the sides of the bowl. Add another 1-2 tbsp of flour if the dough is still too sticky after 10 minutes.
4. Coat a large bowl with oil and scoop the dough inside. Cover in plastic wrap and refrigerate overnight. The dough won’t double in size so don’t worry if it looks like yours hasn’t grown much.
5. When your dough is ready, coat at 9×5 1 lb loaf pan with oil and line the bottom and long sides with one long strip of parchment paper. This makes it easier to take out of the pan.
6. Take the dough out of the fridge and roll it out onto a well-floured surface to 10-inches wide (the side closest to you) and 12-inches long (away from you). Dust the top of the dough with extra flour as needed to prevent your rolling pin from sticking. 
7. Spread the blueberry filling in an even layer over the dough, leaving 1/2 inch all around the outside. If the filling is too sticky and thick, microwave it for about 30 seconds before spreading on top. Also make sure you spread a thin layer, as a thick layer will prevent the dough from baking all the way through.
8. Brush the end farthest away from you with water. Roll the dough up into a log (like making cinnamon rolls). 
9. Gently cut the log lengthwise and lay one piece on the top of the other to form an X (not a +, but an X). Make sure the cut sides are facing up. Working on one side of the X, twist the two pieces together twice, still making sure the cut sides are facing up. Do the same to the other side of the X. You should be left with a twisted log with the blueberry layers facing up (see step-by-step photos in post for reference).
10. Place the log into the bread pan and cover it with a damp tea towel. Leave it to rise for 1 1/2 hours at room temperature.
11. To bake, preheat the oven to 350F. Remove the tea towel and bake on the middle rack for 40-45 minutes. If it starts to brown too quickly around 30-35 minutes, cover it with foil. When inserting a toothpick and you feel a bit of a give or stretch, that means it isn’t done. When it’s done, you’ll feel almost no resistance. 
12. Right when you pull the babka from the oven, make the simple syrup. Add the sugar and water to a heat proof dish and microwave for 1 minute, or until the sugar is completely dissolved.
13. When the babka is still hot, brush the simple syrup all over. Don’t skimp on this, make sure to use up every last drop. Let it cool half in the pan, and half on a cooling rack before eating.
14. When it’s cooled, you can also top it with icing if desired. Simply whisk together the sugar and milk until you reach a glue-like consistency. Drizzle it on top and enjoy!