Preheat oven to 350℉
If you haven't yet, melt your butter. My favorite way is to microwave it in 15 second increments and stirring it when it just starts to melt.
In a large mixing bowl, mix together melted butter and brown sugar. Mix until it is all one color.
Then add the eggs and vanilla extract. Mix again until it is all one color making sure any clumps are crushed and integrated.
Finally, add the dry ingredients: all-purpose flour, baking powder, and salt. This is where things will get tough if mixing by hand. Just keep going and trust the process.
Once the batter is all one uniform color, pour it out into a greased or parchment lined 9"x13" baking pan.
Bake in the oven at 350℉ for 25 - 30 minutes until golden brown. The mixture will deflate when it begins to cool.
Let it cool a little bit before slicing into bars.
