Passing method to component

This is because the Click event of btnClick isn't of the type Action<int> but actually EventCallback<TIn>. So change you'll need to change a few things around.

change ExternalMethod to

[Parameter]
protected EventCallback<int> ExternalMethod {get; set;}

and change the btnClick to

public void btnClick(int param) { /* code */ }
// and then set the razor to
<TestMethodPassing ExternalMethod="@btnClick"></TestMethodPassing>

// Or do it with a lambda in the razor

<TestMethodPassing ExternalMethod="@((param) => {/* code */ })"></TestMethodPassing>

There is a GitHub issue tracking the new Event Handling and Binding here

Here is an example of passing a method from a parent to a child and the child invoking it. As you don't require a return value I'm just using Action rather than Action<T>.

There are many ways you can make this code more compact, but I've gone for a more verbose example to hopefully show what's happening a bit better.

Parent Component:

@page "/"

<h1>Hello, world!</h1>

Welcome to your new app.

<Child ParentMethod="@SayHello" />

@code {

    private void SayHello()
    {
        Console.WriteLine("Hello!");
    }

}

Child Component:

<h3>Child</h3>

<button @onclick="@InvokeParentMethod">Click Me!</button>

@code {

[Parameter] public Action ParentMethod { get; set; }

private void InvokeParentMethod()
{
    ParentMethod?.Invoke();
}

}