Preheat your oven to 350°F (175°C). Grease and flour a 9x5-inch loaf pan or line it with parchment paper.

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. Then, mix in the lemon zest and vanilla extract.

Gradually add the dry ingredients to the butter mixture, alternating with the buttermilk and lemon juice, 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.

Let the cake cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.

For the glaze, whisk together the powdered sugar, lemon juice, and lemon zest in a small bowl until smooth. Drizzle over the cooled cake.

Serve the cake at room temperature.