Preheat the oven to 400°F (200°C). Brush the insides of two ramekins with melted butter; sprinkle with 1 tablespoon sugar. Place on a baking sheet lined with aluminum foil.

Melt butter in a saucepan over medium-low heat; cook and stir flour until golden brown and fragrant, about 2 minutes. Pour in milk and stir until smooth and thick, about 3-4 minutes. Transfer to a large bowl and stir in orange liqueur, zest, egg yolks, and vanilla extract.

Whisk egg whites in a separate large bowl until frothy. Slowly add sugar in increments and continue whisking until the meringue holds its shape but is not stiff.

Fold half the meringue into the egg yolk mixture until combined. Gently fold in the remaining meringue until well mixed. Transfer to prepared ramekins, leaving 1/4-inch space at the top.