Preheat your oven to 425°F (220°C) and grease four ramekins with butter, then dust with cocoa powder to prevent sticking.
In a microwave-safe bowl, combine the semi-sweet chocolate chips and unsalted butter. Melt them together in the microwave in 30-second intervals, stirring after each until smooth and fully combined.
In a separate mixing bowl, whisk together the eggs, granulated sugar, vanilla extract, and salt until the mixture is pale and slightly frothy.
Pour the melted chocolate mixture into the egg mixture and gently fold until well combined, being careful not to overmix.
Sift the cocoa powder over the batter and fold it in until just combined. The batter should be smooth and glossy.
Divide the batter evenly among the prepared ramekins, filling each about ¾ full.
Place the ramekins on a baking sheet and bake in the preheated oven for 12-14 minutes or until the edges are firm but the center still has a slight jiggle.
Remove from the oven and let them cool for 1-2 minutes before carefully inverting each ramekin onto a serving plate. The lava cake should release easily.
Dust with powdered sugar if desired and serve immediately, ideally with a scoop of vanilla ice cream for added indulgence.