Preheat the oven to 300 degrees F (150 degrees C). Grease a 9x12-inch rimmed baking sheet.

Make brownies: 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 large bowl. Add chocolate mixture to sugar mixture; mix until well combined, 4 to 5 minutes. Mix in eggs and vanilla extract. Pour batter into the prepared baking sheet. Sprinkle walnuts on top, pressing them down slightly into batter.

Bake in the preheated oven until edges start to crisp and brownies rise about 1/4 inch, 30 to 40 minutes.

Make glaze: Mix together water, preserves, and unflavored gelatin in a saucepan over high heat; bring to a boil. Cook, stirring occasionally, until thickened, about 2 minutes.

Allow brownies to cool for 30 minutes, then spread a thin layer of glaze on top with a pastry brush.