Preheat oven to 350 degrees F. Line two 8x8 or 9x9 inch baking pans with parchment paper or spray them with nonstick spray.
In a large bowl, whisk together flour, baking powder, and salt. Set aside.
In the bowl of a stand mixer, use the paddle attachment to combine butter, sugar, and brown sugar until pale and fluffy.
Add eggs, vanilla, and sour cream and beat until combined.
Add half of the flour mixture to the wet ingredients. Followed by half of the apple cider. Beat until just combined. Repeat with the other half of flour, then the other half of cider. Do not over-mix.
Add the dates to a small bowl and cover them with warm water to rehydrate them. Set the bowl aside and allow the dates to soak for about 10 minutes.
Peel and dice the apples. Then, use a rubber spatula to fold them into the cake batter. Set the bowl with the batter aside.
Prepare the streusel by cubing room temperature butter into a large mixing bowl.
Add flour, brown sugar, and cinnamon to the butter and use your hands to mix them into the butter until they are well incorporated and form large crumbs.
Drain the water from the dates and remove their pits. Chop the dates into small pieces.
For each pan: pour 2 cups of batter, then 1 cup of streusel, and a ½ cup of dates. Top with 2 cups of batter plus any remaining batter left in the bowl, followed by 1 cup of streusel plus any extra.
Place the baking pans in the oven and bake for 45 to 50 minutes, or until a toothpick inserted in the middle comes out clean with a few crumbs.