It's actually chapter 6.5, so it's right before Yura is introduced.At first I was disappointed that they didn't bring Yura, but then I double-checked and this extra is set when Yura is still at her maximum tsuntsun "I'm gonna kill you" period with Ruruna, and now I'm really disappointed that they didn't bring Yura.
Numbering a chapter that happens after chapter 6 as 11.5 is a ruse so fiendish that only a genius like Ruruna could've come up with it.It's actually chapter 6.5, so it's right before Yura is introduced.