It's, "Te amo con todo el corazón." The fact that it's your heart is understood in the conjugation of the verb (amo), so to say mí corazón is being redundant.
"I love you with all of my heart" in Spanish is "Te amo con todo mi corazón". It is pronounced "Tay AH-mow cone TOE-doe me core-ah-SONE". Please see this site for confirmation of the translation:...