Preheat oven to 350ºF (177ºC) if using a metal pan or 325ºF (163ºC) if using a glass (pyrex) pan.
In a medium saucepan, melt brown sugar (16-ounce) and butter (1 1/2 sticks) together.
In a small bowl, temper eggs (2) with sugar-butter mixture. Add eggs to saucepan.
In a medium bowl, mix together flour (2 cups), baking powder (2 teaspoons), salt (1/2 teaspoon), vanilla (1 teaspoon), and nuts (1 cup).
Add butter mixture to flour mixture. Beat with an electric mixer until combined.
Pour into a well-greased 9x13 pan. Bake at 350ºF (177ºC) for 30 minutes or if using a glass baking dish, bake at 325ºF (163ºC) for 30 minutes.
Notes
Temper the eggs. When mixing warm butter and brown sugar into eggs, always temper first so they don’t scramble—just add a little warm mixture into the eggs before combining everything.
Cool before cutting. It’s tempting to dig right in, but letting them rest for 15–20 minutes helps them set and makes for clean slices.
Storage tip: Wrap leftovers tightly once cooled. The flavor actually deepens after a day, and they stay soft for up to 3 days at room temperature.