Tahin ile pekmezi karıştırınca tatlı bir kahvaltılık oluyor.

Karıştırınca comes from the verb karıştırmak (to mix) plus the suffix -ınca / -ince.

In this sentence, it means something like:

• when (you) mix
• once (you) mix
• sometimes even if (you) mix, depending on context

So Tahin ile pekmezi karıştırınca... means When you mix tahini with molasses...

This suffix often introduces a result or consequence:

• Eve gelince ararım. = When I get home, I’ll call.
• Su ekleyince daha iyi oluyor. = When you add water, it becomes better.

Because the sentence uses the pattern:

X ile Y’yi karıştırmak = to mix Y with X

So here:

• tahin ile = with tahini
• pekmezi = the molasses / molasses as the direct object

The -i on pekmezi is the accusative ending, marking the direct object.

So the structure is literally closer to:

When [someone] mixes the molasses with tahini...

Turkish often treats one thing as the main object and the other as the thing it is mixed with.

Because ile means with, while ve means and.

Here the idea is not just listing two foods, but describing the action of mixing one thing with another:

• tahin ve pekmez = tahini and molasses
• tahin ile pekmezi karıştırmak = to mix the molasses with tahini

In English we often say mix X and Y, but Turkish commonly uses mix Y with X.

Oluyor is from olmak (to become / to be / to happen), and here it means:

• it becomes
• it turns into

So tatlı bir kahvaltılık oluyor means it becomes a sweet breakfast food/item.

Why oluyor instead of olur?

In Turkish, the present continuous form (-yor) is often used for:

• general truths in everyday speech
• results that happen in practice
• recipe-style or explanatory statements

So oluyor sounds very natural in spoken Turkish here:
When you mix tahini with molasses, it becomes a sweet breakfast item.

Olur would also be possible in some contexts, but oluyor feels very idiomatic and conversational.

Kahvaltılık means something like:

• a breakfast food
• something eaten at breakfast
• a breakfast item

It comes from kahvaltı (breakfast) plus the suffix -lık / -lik / -luk / -lük, which often creates words meaning something for, something related to, or something used as.

So:

• kahvaltı = breakfast
• kahvaltılık = something for breakfast

In this sentence, it suggests that the tahini-molasses mixture becomes a sweet thing you eat at breakfast, like a spread or topping.

Here tatlı means sweet as an adjective.

So:

• tatlı bir kahvaltılık = a sweet breakfast item

Although tatlı can also be a noun meaning dessert, that is not what it means here.

Compare:

• Bu tatlı. = This is sweet. / This is a dessert.
  (depends on context)
• tatlı bir şey = something sweet
• tatlı bir kahvaltılık = a sweet breakfast food

Here it clearly describes kahvaltılık.

That is very normal in Turkish.

In karıştırınca, the subject is not stated because it is understood generically. English often has to supply something like:

• when you mix
• when one mixes
• when people mix

In Turkish, that kind of generic subject is often left unstated.

So the sentence is not about a specific person. It means something like:
Whenever you mix tahini with molasses, the result is a sweet breakfast food.

The subject is understood rather than directly stated.

The idea is that the mixture / the result of mixing tahini and molasses becomes a sweet breakfast item.

So the sentence is basically:

When you mix tahini with molasses, [it] becomes a sweet breakfast food.

Turkish often leaves this kind of subject implied when it is obvious from context.

Yes. İle is very often shortened in everyday Turkish to -le / -la.

So this sentence could also be:

Tahinle pekmezi karıştırınca tatlı bir kahvaltılık oluyor.

That means exactly the same thing.

Both are correct:

• tahin ile
• tahinle

The shortened form is extremely common in speech and writing.

A close breakdown is:

• Tahin ile = with tahini
• pekmezi = the molasses / molasses (direct object)
• karıştırınca = when [you] mix
• tatlı bir kahvaltılık = a sweet breakfast item
• oluyor = it becomes

So a fairly literal rendering is:

When [you] mix the molasses with tahini, it becomes a sweet breakfast item.

More natural English would be:

When you mix tahini and molasses, it becomes a sweet breakfast food.

Yes, in some contexts that would work.

The suffix -ınca / -ince usually means when, but depending on context it can feel close to if, especially in practical statements, instructions, or recipes.

So this sentence can suggest both:

• When you mix tahini with molasses, it becomes...
• If you mix tahini with molasses, it becomes...

When is usually the best default translation here, but if can sound natural in English too.