Step 1: Preheat oven to 350°F (175°C) and grease a 9×13-inch baking pan.
Step 2: In a large bowl, cream together softened butter and granulated sugar until light and fluffy.
Step 3: Add eggs one at a time, beating well after each addition, then stir in vanilla extract.
Step 4: In a separate bowl, whisk together flour, baking powder, baking soda, cinnamon, and salt.
Step 5: Alternately add the flour mixture and milk to the butter mixture, mixing just until combined.
Step 6: Fold in mashed sweet potatoes until evenly incorporated into the batter.
Step 7: Pour batter into the prepared baking pan and spread evenly.
Step 8: Sprinkle brown sugar and optional pecans evenly over the top of the batter.
Step 9: Bake for 30-35 minutes or until a toothpick inserted in the center comes out clean.
Step 10: Let cool in pan for 10 minutes before serving warm or at room temperature.