Error defining composite types and returning ROWTYPEs

CREATE TABLE test_table (
   some_column integer,
   another_column text
);

CREATE TYPE test_type as (
   some_type test_table.some_column%TYPE
);

Gives me:
ERROR:  syntax error at or near "%" at character 62
LINE 2:   some_type test_table.some_column%TYPE


I also can't define rowtypes as return types on functions:
CREATE OR REPLACE FUNCTION get_test_table_single()
RETURNS test_table%ROWTYPE AS $$
DECLARE
   v_single test_table%ROWTYPE;
BEGIN
   SELECT * INTO v_single FROM test_table LIMIT 1;
   RETURN v_single;
END;
$$ LANGUAGE plpgsql;

Gives me:
ERROR:  syntax error at or near "%" at character 65
LINE 2: RETURNS test_table%ROWTYPE AS $$

Can somebody tell me what I'm doing wrong? This is driving me insane.

Geoff