Cupcakes: Preheat oven to 350 F. Prepare a cupcake tin and line with cupcake paper liners. Make 1 cup of coffee.
In a bowl whisk together flour, sugar, cocoa powder, baking soda and salt.
In a separate bowl whisk together coffee, olive oil, vanilla extract and vinegar.
Add wet ingredients to the dry ones and with a minimum amount of strokes mix until well combined.
Using an ice-cream scoop transfer batter into cupcake liners. They have to be 2/3 full.
Bake at 350 F for 20 minutes, until toothpick inserted in the center comes out clean. Set on a cooling rack to cool completely.
Prague frosting: In a sauce pan combine egg yolks, water and condensed milk. Whisk together and heat over low to medium heat, constantly stirring until mixture thickens.
Remove from heat and mix in the semi-sweet chocolate. Let cool at room temperature.
In the bowl of a stand mixer cream butter for 5 minutes, until fluffy, stirring the bowl of the mixer. When chocolate mixture is cooled, add it to butter and beat for 2 more minutes.
Fill a piping bag and a tip of your choice, decorate cooled to room temperature cupcakes.