Oatmeal Coffee Cake with Pecan Streusel

Preheat the oven to 375 degrees F (190 degrees C). Grease and flour a 9x12-inch baking dish.

Mix brown sugar, pecans, butter, and flour together in a small bowl for streusel; set aside.

Pour boiling water over oats in a bowl. Combine flour, baking powder, baking soda, and salt in a separate bowl.

Cream brown sugar and butter in a large mixing bowl with an electric mixer until light and fluffy. Add eggs, 1 at a time, beating briefly after each addition. Scrape down the bowl and continue beating until mixture is smooth. Mix in oats, vanilla extract, and cinnamon. Stir in flour mixture with a rubber scraper or wooden spoon. Pour batter into the prepared baking dish and cover with streusel.

Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 35 minutes.