In a medium saucepan, simmer berries and sugar over medium heat until the sugar is melted, and the berries are softened, approximately 15 minutes.
In a small bowl, mix tapioca flour and water.
Pour the tapioca slurry into the simmering berries and stir until thickened. Remove from the heat and allow to cool slightly while preparing the shortbread and streusel.
In a large mixing bowl, cream together the softened butter and sugar.
Add the egg and vanilla. Stir to combine.
Fold in the flour and sea salt to form a soft dough.
Grease a 13x9 baking dish.
Preheat the oven to 350℉.
Gently press the shortbread dough evenly into the 13x9 baking dish. Set the baking dish in the refrigerator while preparing the streusel.
In a small mixing bowl, combine oats, brown sugar, cinnamon, flour, and softened butter.
Using a fork, combine the ingredients to make a clumpy streusel.
Remove the shortbread from the refrigerator and pour the berry compote on to the shortbread. Spread evenly across the top.
Sprinkle the streusel onto the berry compote.
Bake for 30 minutes, or until the shortbread is browning on the bottom and the berry compote is bubbling.
Cool for 1 hour on a cooling rack before icing.
In a small bowl, combine powdered sugar, milk and vanilla extract.
Drizzle the icing over the entire top of the cooled dessert.
Cut into 24 squares.
Can be served immediately and stored at room temperature in an airtight container for up to 5 days.