While the existing answer of Jindrich is generally correct, it does not address the question entirely. The OP asked which layer he should use to calculate the cosine similarity between sentence embeddings and the short answer to this question is none. A metric like cosine similarity requires that the dimensions of the vector contribute equally and meaningfully, but this is not the case for BERT weights released by the original authors. Jacob Devlin
(one of the authors of the BERT paper) wrote:
I’m not sure what these vectors are, since BERT does not generate meaningful sentence vectors. It seems that this is doing average pooling over the word tokens to get a sentence vector, but we never suggested that this will generate meaningful sentence representations. And even if they are decent representations when fed into a DNN trained for a downstream task, it doesn’t mean that they will be meaningful in terms of cosine distance. (Since cosine distance is a linear space where all dimensions are weighted equally).
However, that does not mean you can not use BERT for such a task. It just means that you can not use the pre-trained weights out-of-the-box. You can either train a classifier on top of BERT which learns which sentences are similar (using the [CLS] token) or you can use sentence-transformers which can be used in an unsupervised scenario because they were trained to produce meaningful sentence representations.