Preheat oven to 350° F / 180C (regular bake setting/not fan assisted)
Make the pudding batter: In a medium bowl, mix together the the flour, sugar, baking powder, salt and cinnamon. In a small bowl, mix together the milk, melted butter and vanilla. Add the wet ingredients to the dry ingredients and stir until the batter is smooth. Spread the batter in to the bottom of an 8-cup baking dish. Arrange the apple slices in a single layer of over-lapping apples on top of batter.
Prepare the sauce: In a medium bowl or a 4 cup measuring cup, mix together the brown sugar and flour. Add the boiling water, the butter, vanilla and salt. Stir to combine until butter melts. Lay a regular tablespoon curved side up on top of the apples, near the centre. Slowly pour the sauce over the back of the spoon, letting it run on top of the cake (refer to the step-by-step photos or recipe video above this Recipe Card if this isn't clear). Be sure to leave 3/4-1 inch of room at the top of the baking dish.
Bake in pre-heated 350°F oven for 35-40 minutes or until the cake part looks dry and a toothpick in centre comes out clean when inserted in the top cake part. Pudding will have formed a cake-like top with lots of sauce underneath. Allow to cool for at least 5 minutes, then enjoy warm. Lovely as is, or with a scoop of vanilla ice cream.
Notes
Note 1: A firm baking apple works best, such as a baking apple that you might use for an apple pie. Good baking apples are Granny Smith, Cortland, Braeburn, Fuji, Golden Delicious or Honeycrisp.Refrigerate left-overs up to 2 days or freeze up to 3 months. Re-warm in the microwave or loosely covered with foil in a 350F oven until warmed through.Be sure to read the notes above this Recipe Card, for more tips on making this recipe. You'll also find step-by-step photos and a Recipe Video that you might find helpful.