Melt the butter and white chocolate together in a medium saucepan over medium heat, stirring constantly for about 5 minutes. Or melt in a medium microwave safe bowl in 20 second increments, stirring after each, in the microwave. Remove from the heat, pour into a large mixing bowl, and allow to slightly cool for 10 minutes.

Adjust the oven rack to the lower third position and preheat oven to 350°F (177°C). Line the bottom and sides of a 9-inch square baking pan* with aluminum foil or parchment, leaving an overhang on all sides. Set aside.

Whisk the granulated sugar into the cooled butter mixture. Whisk in the orange juice and zest, then the eggs. Whisk until the batter is completely smooth, then whisk in the vanilla. In a separate medium bowl, whisk the flour, baking powder, salt, and poppy seeds together. Gently fold the dry ingredients into the wet ingredients.

Pour 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. If you find they are browning too quickly on top, loosely cover the bars with aluminum foil. A toothpick inserted into the center of the bars should come out with very few moist crumbs when the bars are done. All ovens are different, so keep an eye on them after 30 minutes and use the toothpick test to see when yours are finished. Allow the bars to cool completely in the pan set on a wire rack.

Combine the glaze ingredients, except for the fresh orange slices, in a medium saucepan over low heat. Whisk until the glaze is smooth. Remove saucepan from heat and either (1) pour glaze over the cooled pan of orange bars, then cut the bars into squares or (2) keep the saucepan of glaze over low heat, cut the orange bars into squares, and dip each individually into the glaze. Garnish each square with orange. The glaze will set after about 10 minutes.