Flour's Protein Content, Yeast + Baking Soda, and Donuts

I tried making some Portuguese donuts (called malasadas). I used an all purpose flour with 11.5 grams of protein per 100 grams of flour (11.5% protein). The dough seemed to rise properly the first and second time. After I fried the dough, the resulting donut seemed a heavy and dense. 

I want the donuts to be very light and fluffy. What can I do to achieve such results. Ideas that come to mind:

• Use a lower protein flour, such as cake flour. Cake flour seems to be hard to find where I'm from. Is there anything I can add to all purpose flour to reduce it's protein profile?

Here's my theory: my 100 grams of flour (a cup of well sifted wheat flour is about 110 grams) contains 11.5 grams of protein. If I subtract 20 grams of flour and replace it with 20 grams of cornstarch (cornstarch has 0% protein), the protein content of my flour/starch mixture should be reduced by 20%. Meaning, I'll have a total of 9.2 grams of protein -- cake flour range. Will this work?

• Use more yeast. If I get more air bubbles in the dough, it might come out lighter. Right?
• Add baking soda to the recipe. The recipe doesn't call for baking soda, but I'm wondering if I can add it to make the donut lighter and fluffier. If this is an option, how do I determine the amount I need?
• Is there something in the kneading I can do to get the desired results? Do I need to knead more or less perhaps?
• Is there anything in the recipe I can change? The recipe (from Food Network):

• 2 teaspoon dry active yeast
• 1/4 cup warm water
• 1 tablespoon sugar plus 1/3 cup sugar
• 2/3 cup milk
• 1 teaspoon vanilla extract
• 4 eggs, well beaten
• 1/2 cup unsalted butter, melted
• 4 cups all-purpose flour
• 1/2 teaspoon salt
• 2 teaspoons freshly grated nutmeg
• Vegetable oil, for frying
• Cinnamon-sugar for coating (about 1/4 cup sugar mixed with cinnamon to taste)