Place the semi-sweet chocolate and butter in a medium glass bowl and microwave for 30 seconds. Remove and stir, and repeat once more. Set aside.
In a small saucepan, heat the heavy cream and corn syrup over medium heat until simmering. Remove from heat and pour over the melted chocolate and butter. Let stand for 2 minutes then use a rubber spatula to stir gently in concentric circles until all chocolate is melted and mixture is smooth and creamy. Gently stir in the vanilla extract, chili powder, cinnamon, and tequila. Pour the mixture into an 8 by 8-inch glass baking dish and refrigerate for 1 hour.
Using a small truffle scooper or melon baller, scoop the chilled chocolate and place onto a sheet pan lined with parchment paper. Chill for 30 more minutes.
Place cocoa powder in a small bowl for dusting. Remove the truffles from the refrigerator and roll into balls between the palms of your hands. Then roll one truffle at a time in the cocoa powder, using a fork to lift the truffle out of the bowl and dust off the excess. Sprinkle with additional chili powder, for garnish. Let rest at room temperature for one hour before serving.
Keep in an air-tight container at room temperature.