Preheat the oven to 350°F. Grease and flour a 9x5-inch loaf pan.
In a medium bowl, whisk together the flour, poppy seeds, baking powder, baking soda, and salt. Set aside.
In a large mixing bowl, cream the softened butter and granulated sugar together until light and fluffy, about 3-4 minutes.
Add the eggs one at a time, mixing well after each addition. Stir in the lemon zest, lemon juice, and vanilla extract.
Gradually add the dry ingredients to the wet mixture, alternating with the buttermilk, starting and ending with the dry ingredients. Mix until just combined.
Pour the batter into the prepared loaf pan and smooth the top.
Bake for 50-60 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cake to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.