Preheat your oven to 350°F and line an 8x8 baking pan.
In a small bowl, combine your berries. Lightly mash the blueberries and blackberries, then mix in the sugar and water. Set aside.
In a large bowl, cream together the butter, brown sugar, and granulated sugar until light and fluffy. Add the eggs and vanilla, mixing until smooth, then stir in the Greek yogurt.
In a separate bowl, whisk together the flour, cinnamon, baking powder, and salt. Gradually fold the dry ingredients into the wet mixture. Slowly add the milk, mixing just until combined.
Pour the batter into your prepared pan.
In another bowl, mix together the streusel ingredients until a crumbly texture forms. Spoon the berry mixture over the batter, then sprinkle the streusel on top.
Bake for about 40 minutes, or until golden and set.
While the cake bakes, whisk together the icing and set aside.
Let the cake cool for 20 minutes, drizzle with icing, and enjoy every bite!
