Preheat oven to Fahrenheit 350°F
Lightly grease 2 quarter sheet cake pans
Line with parchment paper to fill the bottom of the pan and set aside
Heat butter in a saucepan or heavy skillet over medium heat until butter begins to melt
Continue to heat butter until it turns to a dark brown and fills your kitchen with a nutty almost toffee aroma
Remove from pan and transfer to a shallow bowl and set aside until cooled to room temperature
In a clean medium sized bowl beat brown butter and sugars with an electric mixer until fluffy
Add eggs 1 at a time, beating well after each addition
Beat in vanilla
At low speed, mix in half the flour, baking powder, and salt, just incorporated
Add buttermilk and mix until combined
Add remaining flour and stir until smooth
Transfer batter to cake sheets, dividing the batter between the two evenly
Bake for approximately 30-40 minutes, until golden brown
Let cool in pan completely