Step 1Combine the egg yolks and one-third of the sugar in a bowl and whisk to a light ribbon consistency, about 3 to 4 minutes. Add the flour and whisk it in thoroughly.
Step 2In a pan, heat the milk with the rest of the sugar and the vanilla bean and seeds. As soon as it comes to a boil, pour it onto the egg yolk mixture a few tablespoons to start, then all of it, stirring as you go. Mix well, then return the mixture to the pan.
Step 3Bring to a boil over medium heat, stirring constantly with the whisk. Allow the mixture to bubble, still stirring, for 2 minutes, then tip it into a bowl.
Step 4To prevent a skin forming, cover tightly with plastic wrap on the surface of the creme. Chill in an ice-water bath, then refrigerate overnight. Can be kept in the refrigerator for up to 3 days. Remove the vanilla bean before using.
Step 1Combine the milk, butter, salt and sugar in a heavy saucepan over low heat. Bring to a boil, then remove the pan from the heat. Stir in the flour all at once with a wooden spoon until smooth.
Step 2Return the pan to medium heat for about 1 minute, stirring constantly, to dry out the paste. Tip it into a bowl.
Step 3Add the eggs one by one, beating with the wooden spoon until each egg is fully beaten in. Once they are all incorporated into the mixture, it should be smooth and shiny, and thick enough to pipe. The choux paste is now ready to use. (If you are not using it immediately, brush the surface with egg wash to prevent a crust forming.)
Step 4Pipe small mounds, about 2 teaspoons each, onto a baking sheet lined with parchment paper in staggered rows, using a pastry bag fitted with a half-inch round tip.
Step 5Brush the buns with egg wash. Bake 15 to 20 minutes, until dry and crisp, but soft inside. Cool on a wire rack.
Step 1Heat the oven to 400 degrees. Whip the cream in a chilled bowl with the sugar to stiff peaks and fold into the chilled creme patissiere. Dissolve the coffee in the Drambuie and fold into the mixture.
Step 2Make a small opening in the side of each bun with the tip of a knife or skewer. Using a pastry bag fitted with a plain eighth-inch tip, pipe a generous amount of coffee mousse into each bun.
Step 3To serve, dust half the filled buns with a little powdered sugar and the rest with cocoa. Arrange on individual plates or a platter.