To make the glaze, in a stainless-steel bowl, combine the chocolate, butter and corn syrup. Place the bowl over a pan of simmering water, making sure the bottom of the bowl does not touch the water, and heat, stirring occasionally, until the chocolate and butter melt. Remove from over the heat and whisk until smooth. The glaze should be pourable but not so thin that it will run off the terrine. If the glaze is too thin, let it sit at room temperature for about 30 minutes.