Place butter and sugar in a saucepan on very low heat.
As butter melts, stir until butter is completely melted and sugar is dissolved in the butter. Do not brown the butter.
Remove from heat and add to cocoa and salt, stir till blended.
Add one egg at a time, stirring after each one until each is blended in.
Add flour and baking powder, stir until completely blended. Finally, add vanilla.
For thick 300 calorie brownies, pour or scrape batter into a 9 by 9-inch pan lined with parchment paper, and bake for 45 to 60 minutes. The middle will still be soft when done, but the edges will be set.
When done, sprinkle top with powdered sugar and cut into 16 brownies.
For a thinner version, pour/scrape the batter into a 9 by 13-inch pan lined with parchment paper, and bake for 30 minutes. 1/16 of the pan is about 300 calories.
These are very rich – you can cut into smaller pieces. Just be aware that it will take more than 1 brownie for your 300 calories.