Marshmallow is a soft, sweet food that is used in some cakes, puddings, and sweets.（コウビルド英英辞典より）
A beater is a tool or part of a machine which is used for beating things like eggs and cream.
If someone puts all their eggs in one basket, they put all their effort or resources into doing one thing so that, if it fails, they have no alternatives left.
You can't make an omelette without breaking eggs. - something that you say which means it is difficult to achieve something important without causing any unpleasant effects（THE FREE DICTIONARYより） （→関連記事）