Make the Ganache Centers (Can be done up to 3 days ahead):

1. Create the ganache: Place the chopped chocolate in a heat-safe bowl. In a small saucepan, heat the heavy cream over medium heat until it just begins to simmer around the edges (don’t let it boil). Pour the hot cream over the chocolate and let it sit undisturbed for 2 minutes to soften the chocolate.
2. Combine and chill: Add the butter and whisk until the mixture is completely smooth and glossy. Cover the bowl with plastic wrap and refrigerate for at least 2 hours or until firm enough to scoop. You want it to be the consistency of thick frosting.
3. Form ganache portions: Once chilled, use a small ice cream scoop or spoon to portion the ganache into 6 equal balls (about 1 tablespoon each). Place them on a parchment-lined plate and freeze while you prepare the brownie batter. They should be frozen solid before baking.

Prepare the Brownies:

4. Prep your workspace: Preheat your oven to 425°F (220°C). Generously butter the inside of six 6-ounce ramekins, making sure to coat the sides well. Dust each ramekin with cocoa powder, tapping out any excess. Place the prepared ramekins on a baking sheet and set aside.
5. Melt chocolate and butter: In a medium saucepan over low heat, combine the chopped chocolate and butter. Stir constantly until completely melted and smooth, about 3-4 minutes. Remove from heat and let cool for 5 minutes. The mixture should be warm but not hot.
6. Mix in sugar: Add the granulated sugar to the chocolate mixture and whisk vigorously for about 30 seconds until well combined and slightly glossy.
7. Add eggs: Add the eggs one at a time, whisking well after each addition. The batter should become smooth and shiny. Whisk in the vanilla extract.
8. Incorporate dry ingredients: In a small bowl, whisk together the flour, cocoa powder, salt, and espresso powder (if using). Sift or sprinkle the dry ingredients over the chocolate mixture. Gently fold with a rubber spatula until just combined. Don’t overmix—a few small lumps are fine.
9. Assemble the lava brownies: Divide half of the brownie batter evenly among the prepared ramekins (about 2 tablespoons each). Take the frozen ganache balls and press one into the center of each ramekin, pushing it down slightly into the batter. Cover each ganache ball with the remaining brownie batter, making sure the ganache is completely enclosed. The ramekins should be about 3/4 full.
10. Bake to perfection: Bake for 11-13 minutes. The edges should look set and slightly pulling away from the ramekins, while the centers should still look slightly soft and underbaked. This is key—don’t overbake! The brownies will continue cooking from residual heat.
11. Rest and serve: Remove from the oven and let the brownies rest in the ramekins for 1 minute. This helps them set slightly. Run a thin knife around the edges of each ramekin. Place a dessert plate upside down on top of each ramekin and carefully flip to invert the brownie onto the plate. Gently lift off the ramekin. Serve immediately while the center is still molten.
12. Optional garnish: Dust with powdered sugar, add a scoop of vanilla ice cream, garnish with fresh raspberries, or drizzle with chocolate sauce. Serve with a spoon so guests can break into the center and enjoy the lava flow!