Preheat oven to 350ºF (180ºC). Grease a large loaf pan with butter or spray with cooking spray.
Cream together the butter and sugar in a stand mixer or in a large bowl with an electric hand mixer. Add the vanilla extract and egg. Continue mixing until smooth.
½ cup butter, ¾ cup white sugar, 1 teaspoon vanilla extract, 1 egg
In a separate bowl, mix together the flour, baking powder and salt. Add one half of it to the mixer, and when mostly combined, start pouring in the milk. Once the milk is mostly combined, add the remaining flour.
2 cups ap flour, 2 teaspoons baking powder, 1 pinch salt, ¾ cup milk
For the Filling:
In a small bowl, mix the softened butter, flour, sugar and cinnamon until you get a grainy consistency. Set aside.
3 tablespoons butter, ½ cup flour, ½ cup brown sugar, 1 tablespoon cinnamon powder
For the Crumble:
Mix the softened butter, oats, flour, brown sugar and cinnamon with your fingers or a fork until the crumbs form.
3 tablespoons butter, ¼ cup oats, ¼ cup flour, ¼ cup brown sugar, ½ tablespoon cinnamon powder
To Assemble:
Pour half of the cake batter into the loaf pan and spread it out. Sprinkle in the filling and cover with the other half of the cake batter. Sprinkle the crumble over the top and press lightly so it adheres to the top.
Bake in the preheated oven for 45-50 minutes, until a toothpick inserted into the center comes out clean.
Let cool for 10 minutes before transferring to a baking rack to finish cooling. Slice and serve with a cup of coffee.
Notes
You can store the coffee cake, covered, at room temperature for up to 3 to 4 days.
You can also freeze it for up to 3 months, but make sure to cool it first and cover well. Thaw in the refrigerator.