How to shape the corners of the mobile menu icon

Hi All,
I am very new to the Gutenberg blocks and trying to achieve the attached look for the mobile menu icon. I tried everything but not sure what settings to change in order to get a shape like this.

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https://preview.redd.it/721shcnuf41b1.png?width=131&format=png&auto=webp&v=enabled&s=198ce3c13f1befaf9f08407af5b138d9b027d819

 

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