In a large bowl, cream the butter and sugar together until smooth. Add the eggs, one at a time, beating well after each. Stir in the orange zest, vanilla extract, and sour cream until combined. Sift in the flour, baking powder, and salt; add the poppy seeds, and beat until the batter is thick and smooth.
In a small pot on the stove, combine all of the glaze ingredients. Warm on the stovetop until the sugar is completely dissolved (the mixture will come nearly to a boil, but not quite). Prick the loaf cake all over with a toothpick or shish kabob skewer and slowly pour the glaze evenly over the top of the whole loaf.
Tip: I put my hand on the top of the loaf, turn the pan over, peel off the parchment, and then let the cake cool right side up.