In a small saucepan, dissolve the sugar in 1/2 cup of water, about 4-5 minutes. Toss in the butter and stir until melted, about another minute. Move the pan off the heat and stir in the vanilla and cake batter extracts and the salt. Leftover syrup will keep in the fridge for up to 2 weeks.
In another saucepan, warm the milk on the stovetop until steaming. You can also froth it with your espresso machine or portable milk steamer.
Add 2-3 tablespoons of the cake batter syrup to the bottom of your favorite mug. Stir to combine, then top with milk. Garnish with lots of whipped cream and sprinkles, as desired.