Maple-Glazed Sour Cream Doughnuts with Sugared-Walnut Streusel

1 large egg white

1/4 cup sugar

1 1/2 cups chopped walnut s (about 6 ounces) Doughnuts

3 1/2 cups all purpose flour

1 tablespoon baking powder

1 teaspoon ground cinnamon

1 teaspoon salt

1/2 teaspoon baking soda

1 cup sugar

2 large egg s

2 teaspoons (packed) finely grated orange peel

1/2 teaspoon vanilla extract

1/3 cup melted unsalted butter , cooled briefly

1 cup sour cream Glaze

2 cups powdered sugar

1/4 teaspoon maple extract

5 tablespoons (about) heavy whipping cream

Canola oil (for deep-frying)