Preheat oven to 350°F.
Grease 9x5-inch loaf pan with softened butter and dust with flour.
In medium bowl, whisk together flour, baking powder, and salt; set aside.
Take 1 tablespoon of flour mixture and toss with poppy seeds in a small bowl, set aside.
Melt butter (I use microwave--but be sure not to leave it in too long or it will bubble over).
In food processor, process sugar and zest until combined--just a few short pulses.
Add lemon juice, eggs and vanilla; process until combined, about 5 seconds.
With machine running, add melted butter through feed tube (this should take about 20 seconds).
Transfer mixture to large bowl.
Sift flour mixture over eggs in thirds, whisking gently after each addition until just combined.
Gently fold in poppy seeds until evenly incorporated.
Pour batter into prepared pan and bake 15 minutes.
Reduce oven temperature to 325°F and continue baking until deep golden brown and skewer inserted in center comes out clean (about 35 minutes).
Cool in pan for 10 minutes, then turn onto a wire rack and brush on Lemon Glaze, if desired.
Cool to room temperature.
For glaze: While loaf is cooling, bring sugar and lemon juice to a boil in a small nonreactive saucepan, stirring occasionally to dissolve sugar.
Reduce heat to low and simmer until thickened slightly, about 2 minutes.
After removing cake from pan, poke entire top and sides with a toothpick.
Brush top and sides of cake with glaze.
I like to do this while cake is on cooling rack over sink or waxed paper as it can be messy!
