Combine all of the ingredients in a medium-sized pot and cook over medium heat.
Cook for 30-35 mins, until the jam starts to thicken, then cool completely. The jam will thicken more as it cools.
For the blueberry coffee cake:
Preheat the oven to 350℉ (180℃).
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and granulated sugar until light and fluffy.
Add the eggs, vanilla, sour cream, lemon juice, and zest. Mix to combine.
In a separate bowl whisk all of the dry ingredients together, then add to the wet ingredients in 3 additions, mixing completely after each one.
Add the fresh blueberries and gently fold them into the batter.
Grease a Bundt pan ( 8-cup) generously with baking spray, then put 1/2 of the batter around the bottom of the pan. Spoon in and spread out the 1 cup (340g) of jam. Then layer the second half of the batter on top.
For the blueberry streusel:
Combine all of the streusel ingredients, except the blueberries, in a small bowl. Rub the butter into the sugar, flour, zest, and walnuts with your fingertips. Add the blueberries, toss to combine, then spread the streusel across the top of the cake.
Bake for 55-60 mins. When you remove it from the oven, let it cool on a rack for 25-30 mins, and then remove the cake from the pan.
For the drizzle:
Combine the lemon juice and powdered sugar together in a small bowl and whisk to combine. To create a purple drizzle, separate out half the amount of icing into another bowl and add the blueberry jam to it. Whisk to combine. Drizzle on the top of the cake to decorate.
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Notes
You will have more jam than you need to make this recipe. Use the extra to slather on slices of this cake, top toast, or to make cocktails.