What type of sentence is an éclair—a long, thin, frosted pastry—is traditionally filled with vanilla cream, but its flavor can be enhanced with other fillings, such as chocolate, coffee, strawberry, or pistachio?

This is a compound sentence.
A compound sentence contains at least two independent clauses. An independent clause is a clause that can exist on its own, unlike dependent clauses. In the sentence above, there are two independent clauses:
1. An éclair—a long, thin, frosted pastry—is traditionally filled with vanilla cream
2. (But) Its flavor can be enhanced with other fillings, such as chocolate, coffee, strawberry, or pistachio