Step 1 Bring the milk, two-thirds cup water and the sugar to a boil in a saucepan over medium-high heat. Add the butter and allow it to melt. Remove from the heat. Add the flour all at once and stir until the mixture is completely blended and forms a ball. Return the saucepan to the heat and cook, stirring until the mixture does not separate and the bottom of pan has a thin film on the bottom, about 2 minutes.
Step 2 Remove from the heat and beat in 3 eggs one at a time until completely blended.
Step 3To make the puffs, spoon or pipe about one-fourth cup of choux for each puff onto two greased baking sheets. Beat the remaining egg and brush it over the puffs. Combine the black and white sesame seeds and sprinkle over the top of each puff.
Step 4Bake at 425 degrees for 15 minutes, then reduce the heat to 350 degrees and bake for an additional 15 to 20 minutes until the puffs are doubled in size and golden brown. Remove from the oven and cool on a rack. Make a small slit on the side of each puff to allow steam to escape.