Preheat the oven to 350F.
In the bowl of a mixer, cream together the butter and sugar, scraping down the sides occasionally. Add the vanilla and egg, mix in, then stir in pumpkin.
Sprinkle the baking powder, spices and salt over the wet ingredients and mix in, then stir in flour. Run the mixer on low and pour in the milk. When the milk is mostly incorporated, raise the mixer speed to medium and beat until no lumps remain and mixture is smooth
To make topping: add the butter, flour, brown sugar and cinnamon to a food processor and pulse until coarse crumbs form (You can cut in with forks or a pastry cutter, but I am not patient enough for that!). Conversely, you can combine the dry ingredients in a bowl and use your fingers to pinch in the butter until crumbs form.
Grease a 9x9 baking pan. Pour in ½ of cake batter, and spread to all four corners. Spread the apple butter over the top, then pour in remaining cake batter, and spread carefully across the top, disturbing the filling layer as little as possible (an offset spatula works well for this).
Sprinkle the topping over the top and press lightly so it adheres.
Bake in preheated oven for 50-55 minutes, until a toothpick inserted comes out clean. Cool to warm on wire rack before serving.