Combine the tequila, lime juice, agave, Cointreau, mango, and chili pepper in a blender and process on high seed until fully combined. You will still see little flecks of the chili pepper.
Can be served immediately or refrigerated for a few hours.
When ready to serve, pour the Tajín onto a small plate. Rim two glasses with lime juice, then dip into the Tajín. Pour the margaritas into the glasses and serve.