To make the bars, melt the butter and white chocolate together in a medium saucepan over medium heat, stirring constantly until smooth, about 7 minutes. Remove fromheat, pour into a large mixing bowl, set aside and allow to slightly cool for approximately 10 minutes.
Adjust the oven rack to the lower third position and preheat oven to 350°F. Line the bottom and sides of a 9-inch square baking pan with parchment paper, leaving an overhang on all sides for easy removal once baked. Set aside.
In a medium bowl, whisk the flour, baking powder, salt, and poppy seeds together. Set aside.
Whisk the granulated sugar into the cooled butter mixture (you can keep it in the pan). Whisk in the lemon juice and zest, then the eggs and vanilla. Pour the dry ingredients into the pan as well and fold gently until completely incorporated.
Pour the batter into the prepared baking pan and bake for 35-40 minutes or until the bars are lightly browned around the sides and on top and a toothpick inserted in the center comes out clean. Cool completely on a wire rack before removing.
Once cooled, make the glaze. Combine all of the ingredients in a medium saucepan over low heat and whisk until smooth. Remove from heat and pour over cooled bars or dip each individual bar into the glaze after cutting into squares. Garnish with lemon wedge, if desired. The glaze will set after about 10 minutes.
Store in an air tight container at room temperature for up to one week or store unglazed bars in the freezer for one month. Make the glaze and dip thawed bars in glaze prior to serving.