Preheat oven 450F (235C) and prepare four 3 1/2" diameter ramekins by greasing the bottom and sides with baking spray (or lightly grease and flour). If you don't have ramekins, you can use a 6 count jumbo muffin tin instead (see notes). Set on a baking sheet and set aside.
Combine chopped chocolate and butter in a medium-sized microwave safe bowl. Heat in 20 second increments, stirring very well in between, until chocolate and butter are melted and mixture is smooth.
Add instant coffee and salt and stir well.
In a separate bowl, combine eggs, egg yolks, and brown sugar. Use a whisk to beat vigorously for 15 seconds or until mixture is well combined and foamy on top.
Add egg mixture to chocolate mixture and stir until well-combined.
Sift flour over chocolate mixture and stir until combined.
Evenly divide batter into prepared ramekins or muffin tin. Transfer to 450F oven and bake for 9-11 minutes if using ramekins and 6-7 minutes if using a jumbo muffin tin. Edges should be set and baked and look "cakey", centers should spring back to the touch but will be slightly jiggly and may appear under-baked.
Allow to cool for 5 minutes. Run a knife around the edge of the lava cake to loosen it from its container then carefully (the ramekin will be hot! use an oven mit!) invert onto a plate.
Serve warm (be careful, the "lava" inside is HOT and may need another 5 minutes or longer to cool) topped with a dusting of powdered sugar or a scoop of vanilla ice cream.
*If you don't have ramekins, you can use a 6 count jumbo muffin tin instead (because of the smaller size of the muffin tin cavities, the recipe will then yield 6 cakes). They will need to bake for about 6-7 minutes this way.**I've tried this recipe with semisweet chocolate chips (1 cup) instead. The lava cakes will still turn out but I personally felt they were sub-par in comparison to the ones made with premium chocolate bars.***This doesn't make your lava cakes taste like coffee, it just accentuates the chocolate flavor. Highly recommend!