Prepare Ramekins - grease the interior of the ramekin thoroughly with soften butter. Then lightly dust greased ramekins with flour. Remove any excess flour, or else you risk having a chalky exterior on the final cake.
Chop the chocolate with a serrated knife to chip into small uniform pieces, cube butter into ½ inch place both into a double boiler (or if you don’t have a double broiler, position a glass bowl over a pot of simmering water). Stir often to melt evenly. Remove bowl from heat as soon the mixture is melted. Place onto a towel to cool and remove any condensation from the bottom of the bowl. Careful to not let any water/condensation into the chocolate mixture.
In a separate bowl, beat together eggs, gradually adding the sugar. Whip on medium speed for 2 mins.
Once the chocolate mixture is cool enough to handle, slowly add chocolate mixture to the egg mixture. If you add too quickly it you risk deflating the egg mixture. Once thoroughly combined, stir in the flour and salt until combined.
Pour in mixtures into the ramekins (about 3 oz in each).
(If you aren't ready to serve yet, at this point you can put the prepared ramekins into the refrigerator for later in the day. But let it return to room temperature before you go to the next step)
Put filled ramekins into the oven for about 8 mins. They should be raised on the edges, with small bubbles, but still sunken in the middle. (The longer they cook the less "molten lava" they become)
Take Ramekins out of the oven. Place a plate on the top of the ramekin and then flip the plate/ ramekin over plate-side down. The lava cake will loosen from the ramekin and be left upside down on the plate.
Optional (serve with powdered sugar, or fresh fruit, or ice cream, or whipped cream, or raspberry sauce, ect)