_foo.erb

This is a partial

app.rb

require 'rubygems'
require 'sinatra/base'
require 'padrino-helpers'
require 'erubis'

class SimpleApp < Sinatra::Base
  include Padrino::Helpers

  get "/" do
    erb :demo
  end
end

SimpleApp.run!

demo.erb

Note the explicit engine here will work in edge padrino:

<%= partial("foo", :engine => :erb) %>