The amount of spices and flavorings you'll need to add will depend on the flavor and characteristics of the eggnog you use. So go easy at first and adjust as needed.
Lightly beat 4 eggs in a medium bowl with a whisk or fork, no need to use an electric mixer.
Sitr in 1 cup prepared eggnog, ¼ cup milk, 2 tablespoons sugar, ¼ teaspoon ground cinnamon, pinch of nutmeg or allspice¼ teaspoon vanilla extract and whisk to integrate. If using several bread pieces at a time, transfer the custard mixture to a shallow baking dish.
Heat a large non-stick skillet over medium-low heat and add a dab from the 2 tablespoons unsalted butter.
Dip a slice (or slices if cooking several at a time) of bread into the eggnog mixture, 5-10 seconds on each side.
Cook in the hot skillet for 2-3 minutes per side, or until golden and crisp. Repeat the process with the remaining slices of bread, adding more butter between batches.
Serve immediately, 2 pieces per plate, topped with butter, whipped cream, maple syrup or honey, powdered sugar, extra ground cinnamon and fresh fruit or however you like it.