Brownies:
1 cup (2 sticks) unsalted butter, at room temperature
3 cups firmly packed golden brown sugar
4 large eggs
1 tablespoon vanilla extract
3 cups all-purpose flour
1 tablespoon baking powder
1 teaspoon kosher salt
2 cups chopped pecans, toasted

Icing:
1 cup (2 sticks) unsalted butter
2 cups firmly packed golden brown sugar
4 cups powdered sugar
1/3 cup half and half
1 tablespoon vanilla extract

1. To make the brownies: preheat the oven to 350. Line a half sheet (13X18 inch) pan with aluminum foil and grease with butter or cooking spray.
2. Using an electric mixer fitted with the paddle attachment, beat the 1 cup butter and the 3 cups brown sugar on medium-high speed until fluffy, about 2 minutes. Add the eggs and 1 tablespoon vanilla extract and continue beating for another minute.
3. In a bowl, stir together the flour, baking powder, and salt. Beat the flour mixture into the butter mixture on low speed until incorporated. Stir in the pecans.
4. Pour into the prepared pan, smoothing the top. Bake until the bars are set and slightly puffed, 25 to 30 minutes. Cool completely before icing.

1. To make the icing: In a saucepan set over medium heat, melt the 1 cup butter and 2 cups brown sugar. Once the mixture is lightly bubbling, decrease the heat to medium-low and cook, stirring occasionally for 2 more minutes; set aside.
2. Using an electric mixer fitted with the paddle attachment, beat the powdered sugar, half and half, and vanilla on medium speed until creamy, about 1 minute. Add the melted butter mixture and beat until combined.
3. Pour over the cooled brownies and spread evenly. Let the brownies sit for about 30 minutes to allow the icing to firm up before cutting into about 30 squares.

Left uncut in the pan, the bars will keep at room temperature, covered, for up to 3 days. Cut them the day you will serve them.