The Butler, Attesting is Chapter 130 of the Kuroshitsuji manga.
"Ciel Phantomhive" asserts that he is the true head of the Phantomhive house. Baldroy questions Sebastian Michaelis about "Ciel", who muses that this is the first time he has actually met the latter. He promptly asks Sebastian how dare he "do that" to him on "that day". Sebastian also states that he has never encountered someone like "Ciel", whilst a frantic Finnian exclaims that "Ciel" is lying. Sebastian, however, affirms "Ciel's" statement that he is "Earl Ciel Phantomhive", to the servants' disbelief.
At that moment, Tanaka arrives at the staircase and addresses "Ciel", stating that he was not in his room when he brought his tea. He reveals that he knew the truth all along and was planning to take the secret to his grave; however, he muses that this would now be pointless. He proceeds to relate that fifteen years ago, Vincent Phantomhive married Rachel Phantomhive, with Rachel becoming pregnant in the following year. On December 14, 1875, it was a snowy day, and the servants of the Phantomhive Manor were busily preparing for and holding their breath for "that moment."
Then, a servant notified Vincent and Tanaka that Rachel has given birth and they hurried to her, whereupon they discovered that she had given birth to twin boys.
Tanaka comments that they were akin to two peas in a pod and mirror images of each other, so strikingly alike that no one would be able to distinguish them if they were to switch places.
An amused "Ciel" chides Ciel for confusing the servants by impersonating him. He adds that Tanaka told him that he used losing everything in the fire as an opportunity to hide the fact that he was a twin and calls Ciel a naughty boy for doing so. Ciel is hesitant when "Ciel" asks him why he made such an outrageous lie and states that he will not be angry with him. Ciel, then, mutters that he is "the spare" to "Ciel Phantomhive", heir to Earl Phantomhive.