What the AWS documentation does not tell you when creating an AliasTarget is that the Type has to be set to "A" and not to "CNAME".

gistfile1.json

    "AppDNSRecord" : { 
      "Type" : "AWS::Route53::RecordSet",
      "Properties" : { 
        "HostedZoneName" : { "Fn::Join" : [ "", [ {"Ref" : "HostedZone" }, "." ] ] },
        "Name" : { "Fn::Join" : [ "", [ { "Ref" : "MonitoringCNAMERecord" }, "." ] ] },
        "Type" : "A",
        "AliasTarget" : { 
          "HostedZoneId" : { "Fn::GetAtt" : [ "MonitoringELB", "CanonicalHostedZoneNameID" ] },
          "DNSName" : { "Fn::GetAtt" : [ "MonitoringELB", "DNSName" ] } 
        },
        "Comment" : "Monitoring CNAME alias to ELB"
      }   
    }