• Preheat your oven to Combi Steam, 350°F/180°C, 20% (low) humidity. Grease and line the base of a 9×13-inch rectangular baking pan. Set aside.

Make the crumb topping

• Mix the brown sugar, granulated sugar, cinnamon, ginger and salt together in a medium bowl. Stir in the melted butter then, using a fork, lightly mix in the flour just until it looks crumbly and sandy. Set aside while you make the cake batter.
  1 cup light brown sugar (packed), 1/2 cup granulated sugar, 1 Tbsp ground cinnamon, 1 tsp ground ginger, 1/2 tsp fine salt, 3/4 cup unsalted butter, 2 1/4 cups all-purpose flour

Make the cake

• Whisk the flour, baking soda, baking powder, and salt together in a bowl. Set aside.
  2 1/4 cups all-purpose flour, 1 tsp baking soda, 1/2 tsp baking powder, 1/2 tsp fine salt
• Using a stand mixer fitted with the paddle attachment, cream the butter and sugar together on medium speed until smooth and pale, about 2 minutes. Scrape down the bowl with a spatula about halfway through to make sure it's being mixed evenly.
  3/4 cup unsalted butter, softened, 1 cup granulated sugar
• Add the eggs, sour cream, and vanilla to the mixer bowl. Mix again until everything is combined - it will probably look curdled; that’s ok. Change the mixer speed to low and mix in the dry ingredients. Make sure you don't overmix - you want the batter just smooth and with no streaks of flour remaining. It will be thick, this is normal.
  3 eggs, 1 cup sour cream, 2 tsp vanilla extract
• Toss the chopped apples and cinnamon together in a small bowl.
  2 cups peeled and diced apples, 1 tsp ground cinnamon

Assemble

• Spread the batter evenly into your prepared baking pan and scatter the cinnamon apples over the top. Sprinkle the crumb mixture over everything and very gently press it down using dry hands. You just want it to stick to itself, don't compress it too vigorously!

Bake

• Bake until the cake tests clean in the center with a skewer, about 45-50 minutes. It can be hard to tell when this cake is cooked because of all the crumbs on top, so don't rely on looks alone. If you'd like to bake to temperature instead, an instant-read thermometer in the center of the cake should read around 210°F/98°C.
• Remove the cake from the oven and place it on a wire rack. Allow it to cool in the pan before slicing and serving. It's at its best on the day of baking, when it's cooled to the point of being just barely warm. Leftovers will keep in an airtight container at room temperature for a couple of days, or can be frozen, well-wrapped, for up to a month.