In a large mixing bowl, combine flour, salt, sugar, and baking powder. Cut in cold, cubed butter until the mixture resembles coarse crumbs. Pour in milk and beaten egg, mixing gently. Knead slightly and wrap in plastic; refrigerate for 30 minutes.

While the dough chills, heat a skillet and crumble the sausage into the pan. Cook until browned (5-7 minutes). Mix the sausage with scrambled eggs, cheese, and mustard (if using). Let cool.

Preheat the oven to 400°F (200°C). Roll out the chilled dough to ¼ inch thick. Cut into rectangles of about 4x6 inches.

Place a spoonful of filling on one half of the dough rectangle. Fold the other half over, pressing and crimping the edges with a fork.

Arrange the hot pockets on a lined baking sheet, brush with beaten egg, and bake for 20-25 minutes until golden brown.