Preheat your oven to 350°F (175°C). Grease a standard 9×5-inch loaf pan and line it with parchment paper for easy release.

In a large bowl, beat the softened butter and granulated sugar together for 2 to 3 minutes until light and fluffy.

Beat in the eggs one at a time, mixing well after each. Then, add the sourdough discard, lemon zest, lemon juice, and vanilla extract, mixing until smooth.

In another bowl, whisk together the flour, baking powder, and salt. Gently mix this with the wet ingredients until there are no dry flecks of flour.

Lightly fold the poppy seeds into the batter, mixing just until evenly distributed.

Pour the batter into the prepared loaf pan, smoothing the top. Bake for 55 to 60 minutes until the cake is golden and a toothpick comes out clean. If needed, cover with foil for the last 15 to 20 minutes to prevent over-browning.

In a small bowl, whisk the powdered sugar, lemon juice, and heavy cream until you have a smooth, pourable glaze.

Let the cake cool completely in the pan on a wire rack. Once cool, drizzle the glaze over the top and let it set before slicing.