Use an electric mixer to cream together butter and sugar on medium-high speed until light and fluffy, about 2 minutes.
Add the eggs, one at a time, mixing well after each. Add the sour cream and vanilla extract; mix well.
In a separate bowl, whisk together the flour, baking soda, baking powder and salt.
While the mixer is running on low speed, gradually add the dry ingredients to the wet ingredients. Mix just until combined – do not overmix.
Spread half of the batter in an even layer at the bottom of the prepared loaf pan. Sprinkle half of the cinnamon streusel on top. Spread remaining batter in an even layer on top, then sprinkle with remaining cinnamon sugar.
Bake the bread for 1 hour (for a single loaf), or until a toothpick inserted in the center comes out clean. If you're baking the batter in two smaller loaves, the baking time will be much shorter, so start checking the bread at about 45 minutes. Tent the top of the bread loosely with foil if it starts to get too dark before the center is cooked through.
Let the bread cool in the pan on a wire rack for about 10 minutes, then remove from the pan and cool completely.