Preheat the oven to 350°F. Lightly coat a 9x13-inch pan with nonstick spray or line with parchment paper. Set aside.
Create a double boiler by adding a small amount of water to a small saucepan, then setting a metal bowl over the top. Heat to boiling, then reduce to a simmer. Cut the butter into pieces and break apart the chocolate and add both to the bowl. Melt, stirring often, until completely smooth. Remove from heat and set aside.
In a large mixing bowl, stir together sugar, flour, baking powder, and salt. Pour in the melted chocolate mixture and stir to mix. Continue to stir for about 5 minutes; the mixture will become thick, almost like a dough. This will help blend everything well and cool the chocolate.
Stir in the eggs one at a time, making sure they are fully mixed before adding the next. The batter should now look smooth and silky.
Pour into the prepared pan and spread to the edges and corners. Scatter the chopped walnuts over the top in a single layer, pressing gently into the batter.
Bake for 30 minutes, until slightly risen and edges are starting to pull away from the pan. Do not overbake. Let cool on a wire rack.
For the glaze
If you want to add the glaze, combine the preserves, water, and gelatin in a small saucepan. Heat to boiling, then continue to boil, stirring, for 2 minutes, until uniformly mixed and slightly thickened. Brush on the brownies while they are still warm.
Let the brownies cool completely before slicing and serving. You can speed this up by placing them in the refrigerator for 2 hours.