Preheat the oven to 350°. Combine the first 3 ingredients, set aside.
Place granulated sugar and butter in a large bowl; beat with a mixer at medium speed until well blended (about 3 minutes). Add egg substitute; beat 3 minutes. Beat in sour cream, water, and vanilla.
Combine the flour, baking powder, and salt in a large bowl, stirring well with a whisk. Add the flour mixture to the sour cream mixture and mix until blended.
Reserve 3 tablespoons of the brown sugar mixture in a small bowl and set aside. Sprinkle surface of batter with remaining brown sugar mixture. Gently fold batter 4 times.
Line muffin pans with paper liners. Spoon 1 tablespoon of the batter into the paper lined muffin cups. Tap the muffin pan on the counter to spread the batter out over the bottom of the muffin cup.
Evenly sprinkle the batter with the brown sugar mixture. Spoon the remaining batter evenly over the sugar streusel in the muffin cups.
Sprinkle the remaining 3 tablespoons of the brown sugar mixture evenly over the top of the second muffin batter layer. Bake for 25 minutes or until a wooden pick inserted in center comes out clean. Remove the muffins from pan immediately; place on a wire rack.
Combine the powdered sugar, heavy cream, and vanilla in a small bowl, stirring until smooth. Drizzle the icing evenly over the warm muffins.