Learn Spanish (Latin America): La masa mejora a medida que amasamos.

The phrase a medida que (as) takes the indicative when describing a general truth, a habitual action, or a progression you are currently experiencing. Since this sentence states a factual observation (the dough improves as we knead), we use the indicative amasamos. If you were predicting a future, anticipated outcome, like predicting "the dough will improve as we knead it later", you would use the subjunctive amasemos.

Could I use mientras instead of a medida que in this sentence?

You could, but it changes the focus. Mientras simply means "while" and shows two actions happening at the same time. A medida que emphasizes a parallel progression, meaning the improvement happens step-by-step in direct proportion to the kneading process.

Is it a coincidence that masa and amasamos sound so similar?

Not at all! The noun masa means dough, and the verb amasar (to knead) is derived directly from it. In Spanish, it is very common to create a verb by adding an a- prefix and an -ar suffix to an existing noun.

Why don't we use the reflexive se mejora since the dough is improving itself?