Preheat the oven to 300 degrees F (150 degrees C). Grease a 9x12-inch rimmed baking sheet.
Place butter and chocolate in the top of a double boiler over simmering water. Stir frequently, scraping down the sides with a rubber spatula to avoid scorching, until chocolate is melted, about 5 minutes.
Combine sugar and flour in a medium bowl.
Add chocolate mixture to sugar mixture and mix well, about 4 to 5 minutes. Mix in eggs and vanilla extract.
Pour batter into the prepared baking sheet. Sprinkle walnuts on top, pressing down slightly into the batter.
Bake in the preheated oven until edges start to crisp and brownies rise about 1/4 inch, 30 to 40 minutes.
Mix water, apricot preserves, and unflavored gelatin in a saucepan to make the glaze. Mix thoroughly and bring to a boil for 2 minutes.
Remove brownies from the oven and allow to cool for 30 minutes before spreading a thin layer of the glaze on top with a pastry brush.