The ערוך השלחן ס' תצ"ד writes:
" The Torah calls the Yom Tov "שבועות" because of the שבעה שבועות we counted to reach this Yom Tov. "
The Minhag in many German Kehilos was to bake for שבועות a seven-layer cake for the seven weeks we counted.
The Sefer יוסף אומץ ס' תתנ"ד writes the Minhag was to make a cake/kugel in the shape of a seven-rung ladder. The reason
for it is because, during Matan Torah, Hashem tore open all 7 heavens to show that אין עוד מלבדו.