1. Preheat oven to 350ºF. Line a 6-cup standard-size muffin tin with paper or foil liners.
2. In a medium bowl, whisk together flour, 1/4 cup cocoa powder, baking soda and salt. In a large mixing bowl, whisk together mayonnaise, vanilla and 1/2 cup sugar. Add 1/3 of flour mixture to mayonnaise mixture, then stir in 1/2 cup water. Fold in remaining flour mixture, stirring until batter is smooth. Stir in mini chocolate chips.
3. Spoon batter into prepared muffin tin and bake until cupcakes are firm, 20 minutes. Let cupcakes cool completely in muffin tin.
4. Combine heavy cream, remaining 1 Tbsp. sugar and remaining cocoa powder and whip with an electric mixer on high speed until cream just holds stiff peaks. Top each cupcake with whipped cream and serve.