Preheat: Preheat oven to 325°F (160°C). Grease and flour a 9x5 inch loaf pan.
Cream Butter and Sugar: In a large mixing bowl, cream together the softened butter and sugar until light and fluffy. Beat for at least 5 minutes.
Add Eggs: Beat in the eggs one at a time, mixing well after each addition.
Combine Dry Ingredients: In a separate bowl, whisk together the flour, baking powder, and salt.
Alternate Wet and Dry Ingredients: Gradually add the dry ingredients to the wet ingredients, alternating with the milk (or sour cream), beginning and ending with the dry ingredients. Mix until just combined. Do not overmix!
Add Vanilla: Stir in the vanilla extract.
Pour into Pan: Pour the batter into the prepared loaf pan and spread evenly.
Bake: Bake for 60-75 minutes, or until a wooden skewer inserted into the center comes out clean.
Cool: Let the cake cool in the pan for 10 minutes before inverting it onto a wire rack to cool completely.