Sweet Potato Souffle

INGREDIENTS

• 1 cup brown sugar, packed
• 1 cup granulated sugar
• 2 large eggs
• Pinch of salt
• 1 1/2 teaspoons vanilla extract
• 1/2 cup all-purpose flour
• 1/4 cup butter (1/2 stick), softened
• 1/2 cup butter (1 stick), at room temperature, plus more to grease pan
• 1/2 cup milk
• 1 cup finely chopped pecans
• 5 medium sweet potatoes