Baking – When to add marzipan

It's possible that your new brand of self raising flower has more raising agent. This causes a initial raise to happen faster resulting in large 'bubbles' that are less stable. The cake rises higher and then is more prone to falls.

Fast changes in temperature or air pressure will cause the cake to collapse. That means every time you open the oven, or if your oven isn't very air tight every time you open an close the kitchen door. Loud slams of doors are especially bad.

Higher levels of raising agent are especially an issue if you live at a higher elevation or if there is low air pressure that day.

Your first method -- beating eggs and sugar "to the ribbon" -- is a process of denaturing some of the egg proteins into a lattice that will help capture steam as the batter bakes. Also, this method helps ensure your eggs will be well distributed in the batter, with no stripes of eggwhite anywhere.

Your second method -- beating sugar and softened butter to a pale yellow -- appears to be the creaming method, which uses sugar granules to punch millions of tiny air bubbles into the butter. By using this method, when the batter bakes, you get a more even "crumb" (texture of the holes in the final cake). Though you did not mention it, typically this method is used with recipes calling for baking powder/baking soda (you can use the method without a chemical leavener). Also, I would expect the butter-sugar creaming step to take place before you add your melted chocolate.

Neither method is inherently necessary; if you simply stir all ingredients together at once into a uniform batter, you will still get a tasty product. That said, each technique will tend to lighten the brownie into a more cake-like texture, rather than a dense, fudge-like texture. You can do both, if you like, by dividing your sugar in half.