[SalesForce] Helper Method undefined when called from dynamically created component

Using examples from the developer documentation Dynamically Creating Components, I am attempting to create a button dynamically (which I should be able to select in order to create a new component dynamically).

<aura:component implements="forceCommunity:availableForAllPageTypes" access="global" >

    <lightning:button variant="brand" label="Submit" onclick="{! c.handleClick }" />

    <div>{!v.body}</div>
</aura:component>

Controller.js

({
    handleClick : function(cmp) {
        $A.createComponent(
            "lightning:button",
            {
                "aura:id": "findableAuraId",
                "label": "Press Me",
                "onclick": cmp.getReference("c.handlePress")
            },
            function(newButton, status, errorMessage){
                //Add the new button to the body array
                if (status === "SUCCESS") {
                    var body = cmp.get("v.body");
                    body.push(newButton);
                    cmp.set("v.body", body);
                }
                else if (status === "INCOMPLETE") {
                    console.log("No response from server or client is offline.")
                    // Show offline error
                }
                else if (status === "ERROR") {
                    console.log("Error: " + errorMessage);
                    // Show error message
                }
            }
        );
    },
    handlePress : function(cmp) {
        console.log('pressed');
        helper.helperMethod(cmp);
    }
})

helper.js

({
    helperMethod : function(cmp) {
        console.log('helper triggered');
    }
})

when I call the hendlePress method from the dynamically created component, it is able to corretly call the controller, however, the helper method is 'undefined':

message : "Cannot read property 'helperMethod' of undefined" stack :
"TypeError: Cannot read property 'helperMethod' of undefined…

Why is the dynamically created component able to call the controller method, but the controller method unable to call it's helper?