Place ½ cup brown sugar, ½ cup oats, ½ cup flour, ½ cup cold butter and cinnamon in a medium sized bowl. Using a pastry blender, cut butter into the other ingredients until mixture is crumbly (and butter becomes about the size of peas). NOTE: You may also choose to blend these ingredients until crumbly in a food processor, if desired. Set aside.
Make coffeecake batter in a separate large bowl by combining remaining flour, oats and baking powder. In another bowl, blend melted (remaining) butter, brown sugar, egg, vanilla and milk. Add flour mixture to the butter mixture, and stir until ingredients are fully combined.
Spoon the coffeecake batter into a very well greased (bottom and sides) 10 inch springform pan. Spread batter out to completely cover the bottom of the pan. Place the FROZEN (not thawed) berries on top of the batter, and spread out to evenly cover batter. Sprinkle the fruit with the streusel topping, dispersing evenly over the top of the coffeecake to cover.
Bake coffeecake in a preheated 350° F. oven for 1 hour, or until done in the center. You can test doneness by inserting a toothpick into the center of coffeecake. If it comes out clean, it is done. When done, transfer coffeecake to a wire rack, and let it cool in the pan for 30 minutes. Once cooled, run a butter knife around inside edge of pan, then carefully open pan by releasing latch to remove outside form. After coffeecake has completely cooled, it is ready to serve. Slice into 12 pieces, and enjoy!
Triple Berry Streusel Coffeecake https://www.thegratefulgirlcooks.com/triple-berry-streusel-coffeecake/