Preheat the oven to 350°F (175°C). Grease and flour a 9x5-inch loaf pan.
In a large mixing bowl, whisk together the flour, sugar, poppy seeds, baking powder, baking soda, and salt until well combined.
In another bowl, mix the buttermilk, vegetable oil, eggs, vanilla extract, lemon zest, and lemon juice until smooth.
Pour the wet ingredients into the dry ingredients and stir gently until just combined. Be careful not to overmix; a few lumps are okay.
Pour the batter into the prepared loaf pan and smooth the top with a spatula.
Bake for 50-60 minutes or until a toothpick inserted into the center comes out clean.
Let the cake cool in the pan for about 10 minutes, then transfer it to a wire rack to cool completely.
While the cake is cooling, prepare the glaze by whisking together the powdered sugar, lemon juice, and lemon zest in a small bowl until smooth.
Once the cake is completely cool, drizzle the glaze over the top. Allow the glaze to set for about 15 minutes before slicing.