Pour the heavy cream into a medium saucepan. Add the ground cinnamon, ground red chili pepper, and salt. Warm the cream over medium heat until it just begins to simmer. Stir occasionally to allow the spices to infuse evenly. Remove from heat and let it steep for 10 minutes to deepen the flavors.
While the cream is steeping, chop the dark chocolate finely and place it in a heatproof bowl. Gently melt the chocolate using a double boiler or microwave in short bursts, stirring frequently until smooth and glossy. Set aside.
In a separate bowl, whisk together the egg yolks and pure Mexican vanilla extract until the mixture is pale and slightly thickened.
Slowly pour about half of the warm spiced cream into the egg yolks while whisking constantly. This gradual mixing prevents the yolks from curdling by slowly raising their temperature.
Pour the tempered egg yolk mixture back into the saucepan with the remaining cream. Cook over low heat, stirring constantly with a wooden spoon or heatproof spatula, until the custard thickens enough to coat the back of the spoon (about 170°F or 77°C). Be careful not to boil.
Remove the saucepan from heat and whisk in the melted chocolate until the custard is fully combined and smooth.
Strain the custard through a fine mesh sieve into a clean bowl to remove any lumps or cooked egg bits. Pour the smooth custard evenly into ramekins or serving cups.
Refrigerate the pots de crème for at least 4 hours, preferably overnight, to allow them to set into a silky, spoonable consistency.