I see what the plan is!
Step 1: Get Yukino to eat cookie
Step 2: Yukino passes out due to cookie
Step 3: When Yukino wakes up, convince her that Nobara is her girlfriend but the cookie just made her forget that they were dating
This plan is perfect and has no flaws. NOTHING CAN GO WRONG
Really though I hope whatever plan they came up with helped with their cooking.