(24-count) mini-muffin pan, 18 to 20 (1.25-inch diameter) paper mini-muffin cup liners, recommended: Wilton
For the cupcakes: arrange an oven rack in the center of the oven and preheat the oven to 350 degrees F. Line a 24-count mini-muffin pan with 1.25-inch diameter paper liners and set aside.
In a medium bowl, using an electric hand mixer, beat the egg yolks, vegetable oil, 1/3 cup of the sugar, cocoa powder, vanilla extract, cinnamon, and salt, at high speed until smooth, about 2 minutes. Mix in the melted chocolate chips and stir until smooth (mixture will be thick).
In a separate bowl, using an electric hand mixer, beat the egg whites at high speed until they hold soft peaks, about 3 minutes. With the machine running, gradually add the remaining 1/3 cup sugar and continue to beat until the mixture holds stiff peaks, about 2 minutes. Stir 1/3 of the egg white mixture into the chocolate mixture. Using a spatula, fold the remaining egg white mixture. In batches, sift the flour over the batter and fold it in using a spatula. Fill each paper liner to the top with batter and bake for 12 minutes until puffed. Remove from the oven and let cool for 15 minutes.
For the topping: In a small bowl, combine the melted chocolate chips and oil. Stir until smooth.
Using a fork, drizzle the chocolate mixture over the cupcakes. Allow the topping to harden for at least 1 1/2 hours at room temperature before serving. Store at room temperature in an airtight container.