Preheat oven to 350°F (175°C) and grease a 9x9-inch pan.
In a bowl, whisk together flour, cocoa, baking powder, baking soda, and salt. Set aside.
Cream butter and sugar until fluffy. Add eggs, one at a time, then vanilla.
Alternate adding dry ingredients and sour cream, mixing until smooth.
Fold in milk and chocolate chips.
For the streusel, mix sugar, cinnamon, and flour. Cut in cold butter until crumbly.
Pour batter into the pan, top with streusel, and gently press down.
Bake for 35-40 minutes, until a toothpick comes out clean.
Cool for 10 minutes, slice, and serve.