Pre-heat oven to 350 degrees and grease a 9x5 loaf pan with butter.
In a large bowl, mix together flour, baking powder, baking soda and salt.
In the bowl of a stand mixer or in a large bowl with a hand mixer, beat together the butter and sugar for about 1 minute. Add eggs, one at a time, blending well after eat addition. Mix in vanilla extract and lemon zest.
Mix ⅓ of the flour mixture into the wet ingredients until just combined. Mix ⅓ of the buttermilk in until just combined. Continue alternating flour mixture and buttermilk until everything is just combined.
In a separate medium bowl, mix together brown sugar and cinnamon.
Pour ⅓ of the batter into the prepared loaf pan. Pour ⅓ of the cinnamon mixture on top, followed by another ⅓ of the batter and another ⅓ of the cinnamon. Repeat one more time, ending with the cinnamon sugar on top.
Bake on the bottom rack of your oven for 54-58 minutes, until a toothpick stuck in the center comes out clean.
Let cinnamon loaf cool in pan before removing from pan (run a knife along the edges if necessary) and slicing.