Lemon Blueberry Streusel Bars

Bahan-bahan (24 porsi)

• 1 cup (2 sticks) unsalted butter, (at room temperature)
• 3 cups all purpose flour
• 1½ cups old fashioned rolled oats ((don't use quick cook oats))
• 1 1/3 cups packed light brown sugar
• 1 teaspoon salt
• 1 teaspoon baking powder
• 1 large egg ((separated))
• 1 14 -ounce can sweetened condensed milk
• 1/2 cup freshly squeezed lemon juice
• 2 teaspoons grated lemon zest
• 2½ cups room temperature blueberries, (washed and dried on paper towels)