Preheat oven to 350° Fahrenheit (176° Celsius).
In a large mixing bowl, whisk together the dry ingredients: flour, sugar, baking powder, salt, and poppy seeds.
In a smaller mixing bowl, combine wet ingredients: sourdough discard, milk, vanilla extract, lemon extract, butter, and egg.
Stir wet ingredients into dry ingredients and stir until just moistened.
Pour batter into a lightly greased (or parchment lined) 9-inch by 5-inch baking pan.
Bake at for 50 minutes to 55 minutes. Internal temperature should reach at least 195° Fahrenheit (90° Celsius).
Let cool in pan on wire cooling rack for 15 minutes.
Turn out bread onto wire cooling rack and allow to cool completely.
Mix the powdered sugar and milk to form a glaze. When bread has cooled, drizzle glaze over the crust.