Preheat the oven to 350° F. Line a 9x5 loaf pan with parchment paper, allowing for an overhang.
Beat together the butter and sugar in the bowl of a stand mixer.
Add the eggs, one at at time, beating after each addition.
Beat in the lemon juice, zest, vanilla extract, and poppy seeds.
In another bowl, whisk together the flour, baking powder, and salt.
Add the try mix to the butter mixture, alternating with the sour cream. Mix until just combined.
Transfer to the prepared pan and bake for 1 hour, or until a toothpick comes out clean.
Cool in the pan for 10 minutes and then remove to cool completely on a wire rack.
To make the glaze, sift the powdered sugar into a bowl to remove any clumps. Whisk together the powdered sugar and lemon juice until smooth.
Spoon over the loaf, and let stand for about 30 minutes before slicing.