Apple Streusel Coffee Cake

Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13-inch pan.

Mix brown sugar, flour, and cinnamon for topping in a bowl. Cut in cold butter. Stir in walnuts and apple.

Combine flour, baking powder, and salt for cake in a bowl. Beat white sugar and butter together in a large bowl with an electric mixer until creamy. Add dry ingredients. Mix in milk, egg, and vanilla extract. Spread into the prepared pan and sprinkle streusel topping over top. Swirl a knife through both mixtures.

Bake in the preheated oven until a toothpick inserted into the center of the cake comes out clean, 35 to 40 minutes.