Preheat the oven to 350°F.
Grease an 8-inch square baking pan and line with parchment paper.
Make egg replacement: Put 2 tbsp of ground flax seeds and 6 tbsp of water into a small bowl. Mix, and let sit. (You can also use packaged egg replacer if you prefer.)
In a heatproof bowl over simmering water, melt the chopped baking chocolate and butter together, stirring occasionally until smooth. Remove the bowl from the heat and let cool to room temperature.
Mix the brown sugar and vanilla into the chocolate mixture.
Add the flax eggs and mix well.
In a bowl, sift together flour, baking powder, and salt. Slowly fold the flour mixture into the chocolate mixture until combined.
Mix in the chocolate chips and nuts. Pour the batter into the prepared pan.
Bake for 25 to 30 minutes, until a tester comes out clean.
Remove from the oven and cool for at least 10 minutes before cutting.