cat data/interfaces/hello_world.etk 

widget
{
   type: "window";
   name: "main-window";
   requested-width: 320;
   requested-height: 240;
   title: "Hello World!";
}

widget
{
  type: "hbox";
  name: "button-hbox";
  parent: "main-window";
  homogeneous: 0;
  spacing: 0;
}

widget
{
  type: "button";
  name: "hello-button";
  parent: "button-hbox";
  label: "Push me!";
  packing
  {
    append: 1;
    group: "start";
    fill_policy: "expand_fill";
    padding: 0;
  }
}

evolve_cc hello_world.etk -o hello_world.eet

Then you can use hello_world.eet in your Etk app and avoid writing C code for the GUI / layout.