Preheat your oven to 200°C (400°F). Generously grease four 177ml (6 oz) ramekins with unsalted butter. Ensure the butter coats the entire inside surface, including the rim, to prevent sticking. Dust the inside of each greased ramekin with cocoa powder, tapping out any excess. This creates a non-stick layer and adds a subtle chocolate flavor to the exterior of the cakes.

In a heatproof bowl set over a saucepan of simmering water (ensure the bottom of the bowl does not touch the water), melt the 115g (1/2 cup) of unsalted butter with the 115g (4 oz) of finely chopped bittersweet chocolate. Stir gently and continuously until the mixture is smooth and fully combined. Remove the bowl from the heat and let it cool slightly for about 5 minutes.

In a separate medium bowl, whisk together the 2 large eggs, 2 egg yolks, and 60g (1/3 cup) of granulated sugar until well combined and slightly pale. Do not overbeat; you just want to incorporate them.

Gradually whisk the slightly cooled chocolate mixture into the egg and sugar mixture until smooth and fully incorporated. This is a crucial step to temper the eggs slightly and ensure an even batter.

Gently fold in the 30g (1/4 cup) of all-purpose flour and the 1 pinch of salt until just combined. Be careful not to overmix; stop as soon as you no longer see streaks of flour. Overmixing can develop the gluten, leading to tougher cakes.

Divide the batter evenly among the four prepared ramekins, filling each about two-thirds full. You can prepare these ramekins up to this point a few hours in advance and refrigerate them. If baking from chilled, add 1-2 minutes to the baking time.

Place the filled ramekins on a baking sheet. Bake in the preheated oven at 200°C (400°F) for 10 to 14 minutes. The exact time will depend on your oven and how molten you like the center. The edges of the cakes should be set and slightly puffed, while the center should still look soft and slightly jiggly. A visual cue is to gently shake a ramekin; the center should wobble.

Carefully remove the ramekins from the oven. Let them cool for exactly 1 minute. This brief resting period helps the cakes set just enough to be inverted without collapsing entirely.

To serve, carefully run a thin knife around the edge of each ramekin to loosen the cake. Place a dessert plate upside down over the ramekin, and then quickly and confidently invert them together. Gently lift off the ramekin. If the cake sticks slightly, gently tap the bottom of the ramekin.

Dust the lava cakes with powdered sugar, if desired. Serve immediately with fresh berries, a dollop of whipped cream, or a scoop of ice cream for the ultimate New Year's dessert experience.