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To prepare cupcakes: Preheat oven to 350°F. Line 12 (1/2-cup) muffin cups with paper liners.
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. 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.