Preheat the oven to 170ºC (340ºF) and line two 6-inch cake pans with parchment paper.

In a large bowl, sift together flour, cornstarch, granulated sugar, brown sugar, cocoa powder, baking powder, baking soda, and salt.

In a separate bowl, whisk together vegetable oil, sour cream, egg, vanilla extract, and warm coffee or water until well combined.

Pour the wet ingredients into the dry mixture, gently folding until just combined.

Divide the batter evenly between the cake pans and bake for 22-24 minutes.

Let the cakes cool in the pans for about 5 minutes, then turn them out onto a wire rack to cool completely.

Melt the dark chocolate on low heat, then allow it to cool. Beat together butter, powdered sugar, and cocoa powder until fluffy.

Add the cooled chocolate to the butter mixture along with vanilla extract and mix until velvety.

Assemble the cake by layering the cake and frosting, then frost the entire cake.

Decorate with sprinkles before serving.