Is the word interessantíssima the same in Brazilian and European Portuguese?

Yes

The spelling, meaning, and grammatical function of "interessantíssima" (the absolute superlative of interessante) are identical in both Brazilian and Continental Portuguese. The only difference lies in pronunciation:

• Brazilian Portuguese: The vowels are generally more "open" and clearly articulated. The rhythm is more syllable-timed, meaning each syllable receives a relatively similar duration, making the word sound more melodic and the vowels easier to distinguish.
• Continental Portuguese: The pronunciation is characterized by significant "vowel reduction." Unstressed vowels (such as the "e" in the middle of the word) are often swallowed, shortened, or pronounced as a very subtle, neutral sound. The rhythm is more stress-timed, making the word sound more "clipped" or rapid.

Brazilian Portuguese Examples

1. Essa palestra foi interessantíssima! (That lecture was extremely interesting!)
2. Achei a ideia dela interessantíssima. (I found her idea very interesting.)
3. Essa série da Netflix é interessantíssima. (This Netflix series is extremely interesting.)
4. A aula de hoje foi interessantíssima, aprendi muito. (Today's class was very interesting; I learned a lot.)
5. A exposição de arte está interessantíssima. (The art exhibition is extremely interesting.)

Continental Portuguese Examples

1. Esta conferência é interessantíssima. (This conference is extremely interesting.)
2. Achei a proposta interessantíssima. (I found the proposal very interesting.)
3. A leitura deste livro é interessantíssima. (The reading of this book is extremely interesting.)
4. A peça de teatro foi interessantíssima. (The play was extremely interesting.)
5. A explicação do professor foi interessantíssima. (The teacher's explanation was very interesting.)