How do I use nested blocks passed to a method? (metaprogramming?)

Hi guys! So i am totally lost with something: I need a class that creates a pseudo xml output (only nodes). This class has a builder method (a class level method) which receives a block, and according to the block it generates an output.
For example:

XmlBuilder.build { |doc| doc.my_node "2" }

returns this string:

<my_node>2</my_node>

and

DocumentBuilder.build do |doc| 
  doc.outer_node do |outer_node_builder|
    outer_node_builder.inner "2"
  end
end

returns

<outer_node><inner> 2</inner></outer_node>

I think I should override method_missing to get the name of the method (in the 1st example wuold be "my_node" and the args ("2" in the same example), but I cannot do override it.
And more important, I have no idea how to loop through the "nested" block in example 2.
If anyone has a clue, I would really appreciate your insight.
Oh, my class is written like this:

class DocumentBuilder
  def self.build
    if block_given? 
      yield
    else
      "A block should be passed to the builder"
    end
  end

  def method_missing(m, *args, &block)
    puts "override method missing"
    puts m
    puts args
    puts block
  end
end