Adjust the oven rack to the middle position and preheat the oven to 350 F. Fill mini-muffin tins with cupcake liners and spray the liners with baking spray.
Whisk the flour, baking powder, baking soda, salt and 1 1/2 cups cane sugar together in a large bowl. Whisk together the melted butter, buttermilk, oil, vanilla extract, vanilla seeds and egg yolks in a medium bowl.
In a clean bowl of a stand mixer fitted with the whisk attachment, beat the egg whites at medium-high speed until foamy, about 30 seconds. With the machine running, gradually add the remaining 1/4 cup sugar. Continue to beat until stiff peaks just form, 30 to 60 seconds. Transfer to a separate bowl and set aside. Wash the bowl and dry.
Add the flour mixture to the now-empty mixing bowl. With the mixer running at low speed with the whisk attachment, gradually pour in the butter mixture and mix until almost incorporated (a few streaks of dry flour will remain), about 15 seconds. Stop the mixer and scrape the whisk and sides of the bowl. Return the mixer to medium-low speed and beat until smooth and fully incorporated, 10 to 15 seconds.
Using a rubber spatula, stir one-third of the whites into the batter to lighten. Then add the remaining whites and gently fold into the batter until no white streaks remain.
Fill the mini-muffin tin liners three-quarters full and bake until just firm, 8 to 12 minutes. Let cool, and then frost with Creamy Chocolate Frosting.
Add the butter, sugar, cocoa and salt to a food processor. Process until smooth, scraping the sides of the bowl once or twice, about 30 seconds. Add the corn syrup, vanilla and espresso powder and process just until combined, about 10 seconds. Scrape the sides of the bowl one more time, and then drizzle in the chocolate. Pulse until the frosting is creamy, about 15 seconds.