1 cup hot, unseasoned mashed potatoes (no lumps!)
1 cup lukewarm water
2 cup sifted all-purpose flour
3/4 cup unsweetened Dutch process cocoa powder
2-1/4 tsp. baking powder
tsp. Optional: 2 instant coffee granules
1/2 tsp. baking soda
Pinch of salt
cup Optional: 1 miniature semisweet chocolate chips
2/3 cup unsalted butter, softened
2 cup granulated sugar
1 tsp. vanilla
4 eggs, graded "large"
Optional for serving:
Adjust rack to center of oven; preheat oven to 350 degrees F. Grease a 13 by 9 inch baking pan, at least 1-1/2 inches deep, with solid vegetable shortening. Lightly flour the pan, knocking out any excess flour; set aside.
Place mashed potatoes into medium bowl. With small whisk, gradually stir in water to form a smooth mixture; do not beat mixture excessively. Cool to lukewarm.
Meanwhile, sift together flour, cocoa powder, baking powder, optional coffee granules, baking soda, and salt. If using the miniature chocolate chips, place them in a small bowl with about 1 Tbsp. of the sifted dry ingredients and stir well until chips are coated. Set aside.
In large bowl of electric mixer (fitted with paddle beater, if available) combine butter, sugar, and vanilla. Beat at a low speed to blend, then beat 2 minutes at medium speed, scraping down bowl and beater(s) with rubber spatula once or twice. Add eggs, 2 at a time, beating in at a low speed until blended. Scrape bowl and beater(s) with rubber spatula. Increase mixer speed to medium; beat 1 minute.
At lowest speed, add sifted dry ingredients in 3 additions and mashed potato-water mixture in two additions, beginning and ending with dry ingredients and beating after each addition just until blended. Scrape bowl and beater(s) occasionally with rubber spatula. Batter may still appear curdled after all ingredients have been added--OK. Remove bowl from mixer.
If using the chips, add them now, along with any remaining sifted dry ingredients in the bowl. Fold in with rubber spatula only until evenly distributed.
Turn batter into prepared pan. Spread level, making sure batter is well-pushed into corners, then run batter slightly higher along pan edges and in corners.
Bake in preheated oven 27 to 32 minutes, turning pan back-to-front once about halfway during baking. Cake is done when toothpick inserted near center emerges with a few moist crumbs still clinging to it. Do not overbake! Remove to cooling rack.
During baking, cake center will rise higher than edges, but center will fall slightly as cake cools. Cool completely before serving. If desired, sift confectioners' sugar over top just before serving. Store at room temperature, covered airtight, for up to 3 days; freeze for longer storage.