This search is done in a couple steps ...

Go to the Tools ... Developers Tool Kit ...

You see you are looking for the image:
button_in_cart.gif

so in the bottom input box enter:
button_in_cart.gif

and select Catalog then click SEARCH ...

Now, you see the define for this image uses:
BUTTON_IMAGE_IN_CART

so in the bottom input box enter:
BUTTON_IMAGE_IN_CART

and select Catalog and click SEARCH ...

This will show you all the files using this constant ...

Now, look for the code you are trying to change, and you will see similar code with at least one <br /> which should lead you to where the doubles are created based on the code ... when in double, edit the code and change it to something like '<br />xxx' to see where the <br /><br /> is happening ...