Learn Portuguese (Portugal): Põe apenas uma pitada de canela, senão a sobremesa fica muito amarga. - Put only a pinch of cinnamon, otherwise the dessert gets very bitter.

Written as one word, senão means "otherwise" or "or else", introducing a negative consequence or alternative. If it were two words (se não), it would mean "if not" and would need to be part of a full conditional clause with a verb, such as se não puseres apenas uma pitada... (if you don't put just a pinch...).

Is Põe the tu or você form of the command?

It is the informal tu command. For most verbs, the affirmative tu command is identical to the normal "he/she" present tense form (ele/ela põe). If you were giving this instruction to someone you address formally as você, you would use the subjunctive form and say Ponha apenas uma pitada...

Why use fica instead of é or vai ser to say the dessert "gets" bitter?

Using the verb ficar is the most natural way in Portuguese to express a result or a change of state. While ser describes a permanent characteristic, ficar shows how the dessert "turns out" or "becomes" as a direct consequence of adding too much cinnamon.

Does uma pitada only apply to food?

It literally means "a pinch" for cooking ingredients like spices or salt, but just like in English, it is often used metaphorically. For example, you might describe a comment as having uma pitada de ironia (a pinch of irony).