Preheat the oven to 350ºF. Butter a 1.5L baking dish (8x8” square or 9” round). In a large bowl, whisk together flour, ½ cup sugar, baking powder, baking soda, and salt. In a separate bowl, whisk together sour cream, melted butter, eggs, and vanilla extract. Pour sour cream mixture into the dry mixture and stir until just combined (batter will be fairly stiff). Spread into prepared baking dish.Give the batter bowl a quick rinse, then add the berries and toss with the remaining tablespoon of sugar. Scatter berries over cake batter.  To make the streusel, stir together the butter, sugar and flour in a small bowl (I like to work it together with my hands). Sprinkle it evenly over the berries.Bake the cake for 45-55 minutes, until centre feels slightly springy when pressed or a tester comes out clean. (If the streusel is getting dark before the cake is done, loosely cover with foil and finish baking.) Cool in the cake pan on a wire rack.