Preheat your oven to 350°F (175°C).
Prepare a 9x5 inch loaf pan by greasing it lightly with butter or lining it with parchment paper.
In a large mixing bowl, combine the mashed sweet potatoes and softened butter, mixing until smooth and creamy.
Add brown sugar and mix until well blended.
Beat in the eggs and vanilla extract until combined.
In a separate bowl, whisk together the flour, baking powder, baking soda, cinnamon, and salt.
Gradually add the dry mixture to the wet mixture, stirring gently until just combined.
Pour the batter into the prepared loaf pan, smoothing the top with a spatula.
Bake in the preheated oven for 50-60 minutes, or until a toothpick inserted into the center comes out clean.
Once baked, remove the pan from the oven and let it cool in the pan for about 10 minutes before transferring to a wire rack to cool completely.