To prepare cupcakes: Preheat oven to 350 degrees F. Line 12 (1/2-cup) muffin cups with paper liners.Advertisement
Place potato in a saucepan, add water to cover and bring to a boil. Cook until tender, 8 to 10 minutes. Drain and mash until very smooth. Measure out 3/4 cup and let cool slightly. (You may have some potato left over.)
Whisk whole-wheat flour, cake flour, baking powder, baking soda and 1/2 teaspoon salt in a medium bowl.
Beat granulated sugar and oil in a large mixing bowl with an electric mixer on medium speed until combined. Beat in egg, 1/2 teaspoon extract and the 3/4 cup mashed potatoes until combined. With the mixer on low, alternately mix in the dry ingredients and buttermilk, starting and ending with dry ingredients and scraping the sides of the bowl as needed, until just combined. Fold in 1 cup blueberries. Divide the batter among the prepared cups (they will be full).
Bake the cupcakes until a toothpick inserted into the center comes out clean, 22 to 24 minutes. Transfer to a wire rack and let cool completely.
To prepare frosting: Once the cupcakes are cool, bring 2 inches of water to a simmer in the bottom of a double boiler (see Tip). Combine 3/4 cup granulated sugar and 3 tablespoons water in the top of the double boiler. Place over the simmering water and heat, stirring, until the sugar is melted, 2 to 3 minutes. Start beating the mixture with an electric mixer on high speed; add reconstituted egg whites, cream of tartar and a pinch of salt. Continue beating until the mixture looks like a glossy, thick frosting, 5 to 7 minutes. Off the heat, beat for 1 minute more to cool. Add 2 tablespoons (or more, depending on desired color) blueberry preserves (or jam) and 1/4 teaspoon extract and beat on low just to combine. Spread or pipe the frosting on the cooled cupcakes and decorate with fresh blueberries on top, if desired.
Make Ahead Tip: Store unfrosted cupcakes airtight at room temperature for up to 1 day.
Equipment: Muffin tin with 12 (1/2-cup) cups, paper liners
Ingredient Notes: Look for whole-wheat pastry flour in the natural-foods section of large supermarkets and natural-foods stores. Store in the freezer.
Pasteurized dried egg whites are a wise choice in recipes that call for uncooked egg whites. Look for brands like Just Whites in the baking or natural-foods section of most supermarkets.
Tips: No buttermilk? You can make “sour milk” as a substitute: mix 1 1/2 teaspoons lemon juice or vinegar to 1/2 cup nonfat milk.
To improvise a double boiler, bring 2 inches of water to a simmer in a large saucepan, set a medium or large metal bowl over the simmering water. Allow at least an inch between the water and the bottom of the bowl.
DIY Muffin Cups: Make your next batch of muffins or cupcakes the ultimate grab-and-go treat by lining your tin with muffin liners. No liners? No problem. Use 5-inch squares of parchment paper, coat each muffin cup with cooking spray, and push each square into the cups using a small can or bottle, pressing the paper up the sides. (It's OK if some of the paper is sticking out over the rim.) Fill each cup as directed.
3 other carbohydrate, 1 fat