In a large mixing bowl, mix the butter, brown sugar, vanilla extract, milk, eggs and bananas. Set aside.
In a medium mixing bowl, mix the flour, baking powder, cinnamon, baking soda and salt.
Slowly mix the dry ingredients into the wet ingredients using a hand mixer until the batter forms.
Grease a crock-pot casserole dish or a 6 qt or larger crock-pot. If using a regular crock-pot, cut a piece of parchment paper and place it on the bottom of the crock covering the bottom and the sides.
Add the batter to the slow cooker.
In a small mixing bowl, mix the ingredients for the streusel with a fork.
Add the streusel topping to the batter in pieces.
Cover and cook on LOW for 2 hours or until the middle has set.
Serve from the crock-pot casserole dish or if using parchment paper, pull up on the sides of the paper and remove the coffee cake from the slow cooker. Slice and serve.
Notes
double the batter batch for thicker pieces of coffee cake.