Preheat oven to 350 degrees. Line a half-sheet (13 by 18-inch) pan with foil and grease with butter or cooking spray.
Beat the 1 cup butter and 3 cups brown sugar using an electric mixer with a paddle attachment until fluffy (approximately 2 minutes).
Add eggs one at a time while beating on medium-low speed and then add vanilla extract. Beat for 1 more minute and scrape down sides of bowl to make sure everything is even in consistency.
Combine the dry ingredients, flour, baking powder, and salt in a medium bowl.
Beat flour mixture into butter mixture at low-speed.
Stir in pecans and pour into prepared pan. Use a spatula to spread the batter evenly.
Bake for 25 to 30 minutes. Bars should be set and slightly puffed. Cool completely.
For icing, melt 1 cup butter and 2 cups brown sugar in a saucepan over medium heat. Once mixture starts to bubble, decrease heat to medium-low and continue to cook for 2 more minutes, stirring occasionally.
Place powdered sugar, half-and-half, and vanilla extract in a mixing bowl and using a paddle attachment beat on medium speed until creamy.
Add melted butter mixture and beat to combine.
Pool over cooled bars and let icing set for 30 minutes before cutting
Recipe adapted slightly from The Pastry Queen Parties