Preheat oven to 350 degrees
In a large mixing bowl, add muffin mix, eggs, oil, and milk, and stir to combine
Pour half of the batter into an 8x8 or 9x9 greased baking dish
Make the filling by mixing ½ cup brown sugar and ¼ teaspoon cinnamon with ¼ cup melted butter
Spread the filling over the top of the batter in the baking dish
Top with remaining half of batter
Make the crumb topping by adding flour, brown sugar, and cold butter cubes to a small mixing bowl
Squeeze the butter with your fingertips. The warmth from your fingers will soften the butter to allow it to combine with the flour and brown sugar. Continue squeezing and mixing the topping until it looks crumbly (pic below)
Add the topping to the baking dish on top of the batter
Bake at 350 degrees for 32-38 minutes
Batter will seem wiggly when it's done, so you'll cover the dish with foil and leave it on the counter for an hour to set
After an hour, the cake will be set and you can dig in!