Preheat oven to 350 degrees Fahrenheit and grease a 9-inch square baking pan.

Simmer 1 cup apple cider in a small saucepan over medium heat until reduced to 1/2 cup, then cool to room temperature.

In a medium bowl, whisk together 2 cups flour, baking powder, baking soda, and salt.

In a large bowl, cream 1/2 cup softened butter and granulated sugar until light and fluffy.

Beat in eggs one at a time, followed by vanilla extract and the cooled apple cider reduction.

Gradually fold in the dry ingredients alternating with the sour cream until just combined.

Gently fold in the diced apples.

Prepare the streusel by combining brown sugar, 1/4 cup flour, and cinnamon, then cutting in cold butter until crumbly.

Spread the batter into the prepared pan and sprinkle the streusel mixture evenly over the top.

Bake for 40 to 45 minutes or until a toothpick inserted in the center comes out clean.

Cool the cake in the pan on a wire rack for 15 minutes before slicing.