Preheat oven to 325F.
Begin making the bread. Cream the butter and the sugar with a hand mixer.
Next, add the rum, lemon zest, lemon extract, vanilla extract, rum, sourdough discard, lemon juice and sour cream.
Then, one by one, add the eggs, mixing in between.
Add the poppy seeds and lightly mix to incorporate.
In a separate bowl, sift the flour, salt and baking powder. Lightly stir to mix.
Add to the wet ingredients. Mix by hand until flour is no longer visible.
Pour batter into a lined loaf pan. Lightly hit against the counter to evenly distribute.
Bake about an hour, checking at the 45-minute mark. When a toothpick comes out clean, but moist, it is ready.
When the bread is nearing ready, make the rum glaze. Add all rum glaze ingredients to a sauce pan over medium heat.
Warm and stir until butter is melted and sugar is dissolved.
When bread is done, poke holes onto the surface with a toothpick and run the toothpick along the edges, to create a small gap in between the parchment paper and the bread.
Place loaf pan onto a baking sheet in case the rum glaze overflows. Pour the glaze evenly all over the bread. Go slow to avoid it overflowing. Let the bread cool for 15 minutes in the loaf pan.
After 15 minutes, let cool for 30 minutes on a lined rack.
While it cools, combine all lemon glaze ingredients into a bowl. Mix well. It should be thick but pourable. If too thick to pour add a tablespoon more lemon juice.
Pour glaze over top, letting it waterfall down the sides. Garnish with lemon slice if desired.