1. Preheat your oven to 350°F (180°C) and generously grease 3-4 ramekins with butter. Dust the insides of the ramekins with cocoa powder, tapping out any excess.
2. In a small saucepan, melt the butter over low heat. Once melted, remove from heat and stir in the milk until combined. Let the mixture cool slightly.
3. In a mixing bowl, whisk the eggs until smooth. Slowly pour the butter and milk mixture into the eggs while whisking continuously to avoid scrambling the eggs.
4. Sift in the flour, bicarbonate of soda, and cocoa powder into the wet ingredients. Gently fold the dry ingredients into the wet until just combined. Do not overmix.
5. Add in the chocolate chips or chopped chocolate and fold them into the batter until evenly distributed.
6. Divide the batter among the prepared ramekins, filling them about 3/4 full.
7. Place the ramekins on a baking sheet and bake in the preheated oven for 12-15 minutes, or until the edges are set but the center is still slightly jiggly.
8. Remove the lava cakes from the oven and let them cool for a minute. Using oven mitts, carefully invert each ramekin onto a plate and gently tap to release the cakes.
9. Serve the chocolate lava cakes warm, either as is or with a scoop of vanilla ice cream or a dusting of powdered sugar.