To make these Lava Brownies gluten free, replace the all-purpose flour with an equal amount of a 1:1 gluten-free flour blend that is designed for baking. Choose a blend that already contains xanthan gum, or add it if needed according to the package directions. Make sure the cocoa powder, chocolate bar, chocolate chips, and vanilla extract are certified gluten free, since some packaged chocolate products may contain traces of gluten or be processed in shared facilities. The texture should remain rich and fudgy, though the brownies may be slightly more delicate when warm, so allow them to cool fully before slicing.