Combine the egg yolk and the condensed milk in a bowl and whisk using a hand mixer on high for about 10 minutes
Meanwhile, brew the French roast coffee
Once done brewing, put aside ½ a cup then divide the remaining coffee between 4 glasses
Add sugar to taste
Once the custard mixture appears foamy and thick,gently dollop the foam equally between the 4 glasses
Top up with the reserved ½ cup of coffee and sprinkle with cocoa powder