Preheat the oven to 325 degrees F. Spray a 9x9-inch baking pan with nonstick spray. Line the pan with a strip of parchment paper so that it covers the bottom and comes up two sides of the pan. Spray the parchment paper with nonstick spray.
In a saucepan over medium heat, melt the butter and continue to simmer until it turns a deep golden brown (3 to 4 minutes- watch it closely and keep stirring every so often so it does not burn); remove from heat and let it cool to room temperature.
Sift together the flour, baking powder and salt. Set aside.
In a large bowl, use an electric mixer to combine the eggs and sugar. Beat on medium-high speed until light and fluffy, about 3 minutes. Mix in the vanilla and browned butter, scraping the sides of the bowl to incorporate all of the ingredients, as needed. Add the dry ingredients, pecans and toffee. Mix just until combined. Scrape the batter into the prepared pan.
Bake 45 to 55 minutes, or until a toothpick inserted into the center comes out clean (don't over bake- start checking on it at 45 minutes... it should feel pretty firm in the center). And it may take up to 60 minutes to get these baked to the right consistency, depending on your oven and the pan you use. Let the bars cool to room temperature before cutting.