In a large bowl, combine cake mix, water, vanilla and cocoa. Beat on low speed for 30 seconds. Beat on medium speed for an additional 2 minutes.: The kitchen will smell faintly of chocolate as the dry ingredients hydrate, a soft cocoa perfume that sets the mood. When you start mixing, listen for a steady motor hum and watch the batter come together from granular to smooth, with small air pockets forming. The reason for starting on low and then increasing the speed is to prevent heavy splatter while still incorporating air; this staged mixing helps the cake rise evenly later. Aim for a batter that is glossy but not runny, and pause to scrape the bowl once so all the dry cocoa is incorporated. A common mistake is overbeating which can deflate the structure, so stop once the batter looks cohesive and has increased slightly in volume.
Gently spoon into an ungreased 10 in. tube pan. Cut through batter with knife to remove air pockets. Bake according to directions on box.: As you transfer the batter, you will notice how light and billowy it feels, almost cloudlike, and spooning it gently preserves that lift. Do not grease the pan because angel food cake needs to cling to the sides as it rises. Running a thin knife through the batter eliminates large trapped air pockets that could create uneven tunnels as it bakes. The surface should be smooth but not pressed down. While the box gives a time and temperature, watch for visual cues, like a domed top that is springy and a slight pullaway from the pan edge. An error to avoid is tapping the pan hard to level the batter, which compresses the air and can lead to a dense cake.
In a small saucepan combining Truvía® Spoonable and cornstarch. Add strawberries. Bring to a boil stirring constantly. Cook and stir about 5 minutes or until the mixture is thickened. Remove from heat and let cool. Puree with a blender.: The sauce starts with a bright, wet aroma as the strawberries heat up, and the mixture will steam lightly as it reaches a simmer. Stirring constantly is important because the cornstarch activates quickly and you want a smooth, lump free texture. As it thickens, the sound changes to a gentle bubbling, and the color deepens to an intense red. After removing from heat, allow it to cool slightly to preserve aroma, then puree until silky; the blender will hum and the sauce will transform to a glossy finish. A typical pitfall is cooking too long, which can dull the fruit flavor, so stop once the sauce coats the spoon and has a slightly viscous sheen.
Run a knife around side and center of cake in pan. Remove to serving platter. Slice and serve with strawberry sauce and whipped topping.: When the cake is done and cooled enough to handle, you will feel the gentle resistance as the knife slips between cake and pan, releasing steam and a faint chocolate note. Removing it to a platter reveals the airy crumb and the tall silhouette of the tube pan bake. Slice with a serrated knife in a gentle sawing motion so the angel food cake keeps its airy texture rather than compressing. Spoon the warm or cooled strawberry sauce over each slice and finish with whipped topping if you like. If the cake sticks, let it rest a bit longer; forcing removal can tear the delicate crust.
