Preheat your oven to 350°F (175°C). Grease and line an 8-inch (20cm) springform cake pan with baking paper to prevent sticking.
In a bowl, mix hot milk and vinegar; let it sit for 5 minutes to create a tangy buttermilk substitute that enhances moisture.
Add instant coffee, oil, vanilla extract, and sugar to the buttermilk mixture; whisk until the sugar dissolves completely and the mixture becomes smooth.
In a separate bowl, sift together all-purpose flour, cocoa powder, cornstarch, baking soda, baking powder, and salt; gradually fold the dry ingredients into the wet mixture, stirring until you achieve a silky, lump-free batter.
Pour the batter into the prepared cake pan, smoothing the top; bake for about 25 minutes, or until the center feels firm to the touch and a toothpick inserted comes out clean.
Let the cake cool completely in the pan before unmolding; for a deeper coffee essence, brush or soak the cake with freshly brewed espresso once cooled.