Preheat oven to 180°C (350°F). Grease 4 little ramekins (I used 4 oz ramekins) 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 prevent the chocolate from burning. You can also melt the chocolate "au bain marie", this method consists 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 the 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 into 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 'cooks' 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 creating a runny lava center is to undercook 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 it off the oven when the center of the cake jiggle and the border are slightly set. 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 keeps baking slightly when it cools down as the ramekin is hot. That is why you must remove the lava cake from the oven when the center jiggles to enjoy a runny lava cake.
Eat directly in the ramekin with a scoop of sugar-free vanilla ice cream on the side or upside down on a plate (careful the cake may break if its center is very runny!)
This recipe serves 4 lava cakes 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 cakes longer, they will make delicious keto chocolate cakes 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, these are already sweet so 1/4 cup of sweetener is enough!