Moving mask- reveals more of image beneath as it grows larger

I am a visual artist with only moderate experience with Premiere and I've never made an animated mask.
I am using Premiere 14.2.0 on a macbook.
I have text that grows from the center of the screen and I want the inside of the letters to be a mask that reveals more and more of an image beneath it.
Let me know if I need to give more info. Any assistant is greatly appreciated. Thanks.