Preheat oven to 180 C (350F). Grease 4 little ramekin (I used 4 oz ramekin) with a bit of butter. Set aside.
In a medium saucepan, place cubed butter and 99% dark chocolate (or any dark chocolate with more than 85%). Melt on low heat, stirring all the time to avoid the chocolate to burn. You can also melt the chocolate "au bain marie", this method consist of placing the saucepan over a bowl of boiling water to melt it gently. When the chocolate and butter are melted remove the saucepan from heat and set aside.
In a small mixing bowl, using an electric whisk, whisk the eggs and sugar-free crystal sweetener on high speed for 1 minute until foamy, fluffy and lighter in color. Note: use 1/3 cup sweetener if your chocolate is unsweetened or 99%, otherwise if you are using stevia sweetened chocolate or 85% cocoa chocolate, use only 1/4 cup sugar-free crystal sweetener.
Stir in (don't whisk!) the vanilla, ground espresso and almond flour.
Gently pour the melted chocolate in the previous egg mixture, stirring at the same time to form the cake batter. Make sure the melted chocolate is not burning hot or it 'cook' the eggs but if you follow the low-heat technique above that will not be a problem.
Fill the 4 ramekins evenly to make sure they bake at the same speed.
Place the ramekin onto a baking tray and bring the tray into the oven.
Bake for 9-12 minutes. The trick to create a runny, lava center is to under cooked the cake. After 9 minutes, you should have a runny lava cake, after 10 minutes a molten lava cake and after 12 minutes a gooey chocolate cake. In any case you can eat the cake but for a lava center you must take off from the oven when the center of the cake jiggle and the border are slightly set. Simply practice to learn how long you need to bake your lava cake to reach your favorite texture.
Cool 2-3 minutes in the ramekin before serving - note that the cake keep baking slightly when it cool down as the ramekin is hot. That is why you must remove the lava cake from the oven when the center jiggle to enjoy a runny lava cake.
Eat directly in the ramekin with a scoop of sugar free vanilla ice cream on side or up side down on a plate (careful the cake may break if center is very runny!)
Net carbs per lava cake : 1.2 gram using sugar free stevia sweetened chocolateThis recipe serve 4 lava cake if you want to eat only 2 of them you can store the uncooked lava cake in the fridge for 24 hours and bake the next day. Otherwise you can bake the 2 extra lava cake longer, they will make delicious keto chocolate cake that you can store for 4 days or freeze and eat for dessert, snack or breakfast.Sweetener amount :if you are using 99% dark chocolate or unsweetened dark chocolate make sure you add 1/3 cup sugar free crystal sweetener. If you use sugar free stevia sweetened chocolate or 85% dark chocolate, those are already sweet so 1/4 cup sweetener is enough !