If the last 4 ingredients need mixing together first, then you simply have a mix column with just those ingredient rows under the mix column from above to show that those get mixed independently in parallel. This recipe implies that is unnecessary for this food though.

It clearly shows the sequential and parallel threads of execution:

• up/down: parallel.
• left to right: sequential

It clearly shows the sequence of produced intermediate results, split and merging points:

• a row that splits into two rows is one step producing more than one result. Like spliting yolk and whites.
• many rows becoming one: merging node. Like baking many ingredients.

Because it fits on a little card. Which everyone used to keep recipes on for a long time. Little boxes full of recipe cards.

For example, the last 4 ingredients should be mixed together independently before folding in to the base.

Isn't that indicated by the combination of rows?

I like this chart. It even allows for ~~multithreaded~~ multi-person baking, because the combination of rows indicates where processes are merged (and need to be done).

Edit: I may be dumb. A mix step is missing below the mix block of the other ingredients, but it could easily be included.