1) Preheat the oven to 350°F. Lightly grease a 9" square pan.
2) To make the streusel: Mix the baking mix, brown sugar, and cinnamon together until evenly blended. Work in the butter until evenly crumbly. Set aside.
3) To make the batter: Whisk the baking mix and sugar together.
4) Whisk the melted butter or oil, eggs, milk or buttermilk, and vanilla together.
5) Add the wet ingredients to the dry ingredients, stirring just until blended.
6) Spread the batter into the prepared pan.
7) Sprinkle the streusel uniformly over the batter, and lightly press some of it into the batter with your fingertips. Pretend you're gently pressing dimples into very soft clay.
8) Bake the cake until a cake tester inserted into the center comes out clean, and the middle springs back when pressed lightly, 22 to 26 minutes. Remove from the oven, and cool for 15 to 20 minutes before cutting into squares.
9) When completely cool, wrap well and store at room temperature for several days. Freeze for longer storage.
Yield: 9 to 12 servings.