Begin by preheating your oven to 425°F (220°C). Lightly grease four 6-ounce ramekins with butter or non-stick spray, then dust them with a little cocoa powder to prevent sticking and add extra chocolate flavor. Set these aside on a baking sheet.
In a microwave-safe bowl or over a double boiler, gently melt the 4 oz bittersweet chocolate and 4 oz butter together. Stir frequently until smooth and fully combined. If using a microwave, heat in 20-second bursts to avoid burning the chocolate. Let the mixture cool slightly.
In a separate mixing bowl, whisk the 2 large eggs and 1/2 cup sugar until the mixture turns pale and slightly thickened. This will help create a light and airy texture in your cakes.
Slowly pour the melted chocolate-butter mixture into the eggs and sugar while whisking continuously. Add 1 teaspoon vanilla extract and, if you’re using it, 1/2 teaspoon instant coffee or espresso powder to deepen the chocolate flavor. Gently fold in 2 tablespoons of flour until just combined — be careful not to overmix.
Divide the batter evenly among the prepared ramekins. Place the baking sheet with the ramekins into the preheated oven and bake for 12-14 minutes. The edges should be firm and set, but the centers will still look soft and slightly jiggly — this is what creates the molten lava effect.
Remove the cakes from the oven and let them cool for 1-2 minutes. Run a knife around the edges to loosen, then carefully invert each ramekin onto a serving plate. Serve immediately, optionally garnished with powdered sugar, fresh berries, or a scoop of vanilla ice cream for a truly indulgent finish.