Separate your egg yolks from your egg whites. Set two bowls out. Crack one egg into your loosely cupped hand over the first bowl. Let the egg white drip through your fingers into the bowl. Place the yolk into the other bowl. Repeat three times. Set aside.
In a large microwave safe bowl, add the butter and chocolate. Microwave 30 seconds, then stir. Microwave 30 more seconds and stir again. Chocolate should be melty with chunks.
Add heavy cream and instant coffee to the chocolate mixture and microwave 30 more seconds. Stir until all the chunks are completely melted and mixture looks glossy. The mixture should be warm. If it is hot, let it cool until it's warm.
Add egg yolks to chocolate mixture and whisk to incorporate. The mixture should have a pudding like consistency at this point. Set aside.
Whip the egg whites in a stand mixer with the whisk attachment on medium speed until whipped and frothy with no peaks, about 1 minute.
Gradually add the powdered sugar to the egg whites and continue to beat on medium speed until you reach stiff peaks, about 2 to 3 more minutes.
Fold one third of the whipped egg whites into the chocolate mixture. Repeat until all the whites are folded into the chocolate mixture evenly.
Transfer the chocolate mousse to a large piping bag and pipe into ramekins. Alternatively, you can use a portion scoop to scoop the mousse into the ramekins.
Cover the ramekins with saran wrap and chill for 4 hours.