Begin by preheating your oven to 350°F (175°C). This ensures that it's ready when your cake is prepared.
Take an 8-inch square pan and lightly grease it with butter or coat it with non-stick cooking spray. This prevents the cake from sticking to the pan once baked.
In a bowl, mix together the "JIFFY" Baking Mix, brown sugar, and cinnamon for the topping. Ensure they are well combined.
Cut the cold butter into small cubes and add them to the dry mixture. Using a pastry cutter or your fingers, work the butter into the dry ingredients until it forms a crumbly texture.
Once the mixture resembles coarse crumbs, add in the chopped nuts (or chopped pecans or raisins if using) and gently mix until evenly distributed. Set the topping aside for later use.
In another mixing bowl, combine the "JIFFY" Baking Mix and sugar for the cake batter. Stir them together until evenly distributed.
Cut the cold butter into small pieces and add them to the dry mixture. Using a pastry cutter or your fingers, blend the butter into the dry ingredients until it resembles coarse crumbs.
In a separate small bowl, whisk together the milk, slightly beaten egg, and vanilla extract until well combined.
Pour the wet ingredients into the bowl with the dry ingredients. Mix them together using a spatula or wooden spoon until just combined. Be careful not to overmix, as this can result in a dense cake.
Pour the cake batter into the prepared 8-inch square pan, spreading it out evenly with a spatula.
Sprinkle the prepared topping evenly over the surface of the cake batter, ensuring it covers the entire surface.
Place the pan in the preheated oven and bake for 25 to 30 minutes, or until the cake is golden brown and a toothpick inserted into the center comes out clean.
Once baked, remove the coffee cake from the oven and allow it to cool slightly in the pan before serving.