Preheat the oven to 350 °F (175 °C). Prepare a 9x9-inch (23x23 cm) loaf pan by greasing and flouring it, then line the bottom with parchment.
To make the streusel topping
In a medium bowl, combine the nuts, brown sugar, flour, oats, and salt.
Drizzle in the melted butter and stir it in with a fork until the mixture resembles a coarse crumble. Set aside.
To make the cake
Slice one of the apples and peel, core, and dice the other one. Set both of these aside (see note).
In a large bowl, combine the flour, baking powder, cinnamon and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and sugar.
Add the eggs one at a time, mixing well between each addition, then add in the apple butter and brandy.
Pour the dry ingredients into the bowl, and stir to just combine, then add the chopped apple.
Transfer the batter into the prepared pan and set aside.
Arrange the sliced apple and sprinkle the streusel topping evenly over the top.
Bake the cake until a cake tester inserted through the centre (not through the fruit if you can avoid them!) comes out clean. This takes about 45 minutes,
Let the cake cool completely before serving.
Notes
Substitutions that will work:
Nuts—I used pecans but you can use chopped walnuts or even chopped hazelnuts. For a nut-free version, use sunflower seeds and/or pumpkin seeds.
Apple butter—replace it with another fruit butter, like pear butter (which is easier to find). You can still incorporate fresh apple with this.
Fresh apples—I've tested this recipe with Cortland apples and Gala apples. Honeycrisp would also work. Use any apple variety that holds up to baking well. Don't use McIntosh, for example, which turns to mush when heated.
If you use a variety of apples that browns when cut, please soak them in a lemon juice bath (dilute lemon juice in water) to ensure they don't brown excessively after cutting. Strain and blot them dry before incorporating in the cake.
Check the cake at a few spots and press the surface gently to make sure the cake has set and springs back when pressed. Even if the edges brown significantly, the inside will not dry out.