In a bowl, using a fork, mix the flour with the baking powder and salt. Mash in the butter until blended. Add the milk and egg and stir until a dough begins to form. Turn the dough out onto a lightly floured work surface and lightly knead it until smooth. Flatten the dough into a disk, wrap in plastic and refrigerate until chilled, at least 1 hour.
Preheat the oven to 400&;176;. Divide the dough into 4 equal pieces. On a lightly floured work surface, roll out 1 piece of dough until it is as thin as possible. Carefully transfer the dough to a large baking sheet and brush it lightly with water. Sprinkle lightly with kosher salt and fennel seeds and bake for 8 to 10 minutes, or until the cracker is puffed, crisp and golden brown. Transfer to a rack to cool. Repeat with the remaining dough, salt and fennel seeds. Break the crackers into pieces before serving.
The crackers can be kept overnight in an airtight container.