1. Heat oven to 350°. Grease inside of a 10-inch cast iron skillet.
2. Combine flour, baking powder, and salt in a medium bowl. Whisk melted butter and brown sugar in a large bowl until combined. Add eggs and vanilla and mix well. Fold dry ingredients into butter mixture until just combined. Spread dough into prepared skillet.
3. To make pecan topping, melt butter in a medium skillet over medium heat, swirling occasionally, until milk solids become nutty and brown. Stir in sugar, salt, and pecans. Pour mixture over blondie dough and spread into an even layer. Transfer to oven and bake until edges are firm and pulling away from the sides of the skillet, 30–35 minutes. Let cool before slicing and serving with ice cream, if desired.