Preheat the oven to 350 degrees F. Line 24 muffin cups with paper liners. Whisk 2 cups flour and 1 teaspoon each baking soda and salt in a medium bowl.
Heat 1 1/2 cups milk in a small saucepan until hot but not boiling; whisk into 1 cup unsweetened cocoa powder in a large bowl until smooth. Let cool slightly. Whisk in 2 cups sugar, 1 1/2 cups vegetable oil, 2 eggs and 2 teaspoons vanilla until smooth. Whisk in the flour mixture until just combined.
Divide the batter among the muffin cups and bake until the tops spring back, 20 to 25 minutes. Let cool 5 minutes in the pans, then remove the cupcakes to a rack to cool completely.