• Position an oven rack in the center and preheat the oven to 350F degrees. Lightly mist an 8 x 8-inch square baking pan or 12-cup muffin tin with nonstick cooking spray and set it aside.
• To make the streusel, in a small bowl, stir together the brown sugar, cinnamon and pecans until well combined.
• To make the cake, in a large bowl, beat together the dry cake mix, flour, yogurt, water, oil, egg and vanilla with an electric mixer on medium until smooth, 1 to 2 minutes.
• Scrape 1/2 the batter into the prepared pan and smooth it out evenly. Sprinkle 1/2 the streusel mixture evenly over the batter in the pan.
• Scrape the remaining batter over the topping, spreading it out evenly. Sprinkle the rest of the streusel topping over the cake batter.
• Bake until golden brown and a toothpick inserted in the center comes out clean, 25 to 30 minutes.
• Remove from the oven and place on a wire rack to cool for 20 minutes.
  
• Slice into 12 even pieces and serve while still a little warm.

To Make Muffins:

• Follow Steps 1 through 4 above (for how to make cinnamon streusel coffee cake). In Step 2, lightly grease a standard muffin pan with nonstick spray and set aside, in place of the square baking pan.
• Scrape half the batter into the prepared muffin pan, using about 1 tablespoon batter in the bottom of each muffin cup. Sprinkle half the streusel mixture (chopped pecans, cinnamon and light brown sugar) evenly over the batter in each cup.
• Optional, to make Raspberry Streusel Coffee Cake muffins, drop about 1 teaspoon raspberry all fruit spread over the batter in each cup. (I added some raspberry fruit spread to six of my muffins for a little variety.) This can be done before or after adding the streusel mixture.
• Spoon the remaining batter evenly over the streusel layer in the 12 muffin cups. Sprinkle the remaining streusel mixture evenly over the 12 muffins.
• Bake at 350F for about 15 minutes, or until a toothpick inserted in the center of the muffins comes out clean and the tops are golden and set.
• Cool muffins in pan until cool enough to handle, then remove the muffins to cooling rack to cool completely. You may have to run a small sharp knife around the edges of each muffin so it releases easily from the pan.