Preheat the oven to 325 degrees F. Generously butter and flour a nonstick muffin tin, or line the cups with large cupcake papers.
Melt the butter in a large saucepan over medium-low heat, swirling until it begins to brown and smell nutty- about 8 minutes. Remove from the heat and immediately stir in the granulated sugar, cocoa, water, vanilla, and salt. Allow it to cool for 5 minutes.
Add the eggs one at a time, whisking well after each addition. Add the flour and mix vigorously for about 1 minute. Stir in the walnuts, if desired (optional).
Spoon the batter into the prepared muffin tin, evenly distributing the batter. It's ok to fill the muffin cups almost to the top edge becuase these brownie lava cakes barely rise. Press some chocolate chips into the center of each brownie to create a chocolate lava center.
Bake for 15-20 minutes, until the edges are set but the middle is still a bit gooey. If need be, cut into one brownie with a paring knife to determine whether the batter is set.
Allow them to cool in the pan for 10 minutes before removing. Dust with confectioner's sugar and serve immediately. A scoop of ice cream is also recommended! Store leftovers in a resealable bag at room temperature for up to 3 days- or freeze for a few months.