Part 1: Prepare the Pecan Crunch Topping

1. Toast the Pecans: Preheat your oven to 350°F (175°C). Spread the pecans in a single layer on a baking sheet and toast for 7-9 minutes, or until they are fragrant and slightly darkened. Let them cool completely, then chop them coarsely. This step is crucial for flavor.

2. Combine Dry Ingredients: In a medium bowl, whisk together the ½ cup of flour, ½ cup of brown sugar, 1 teaspoon of cinnamon, and ¼ teaspoon of salt.

3. Cut in the Butter: Add the ¼ cup of cold, cubed butter to the dry ingredients. Using a pastry blender, two knives, or your fingertips, cut the butter into the mixture until it resembles coarse, damp crumbs with some pea-sized pieces of butter remaining.

4. Add Pecans: Stir in the 1 cup of toasted, chopped pecans. Set the topping mixture aside (you can place it in the fridge to keep the butter cold while you make the cake batter).

Part 2: Make the Brown Sugar Butter Cake

1. Preheat and Prepare Pan: Keep the oven at 350°F (175°C). Grease a 9×13 inch baking pan with butter or baking spray.

2. Combine Dry Ingredients: In a large bowl, whisk together the 2 cups of flour, 2 teaspoons of baking powder, ½ teaspoon of baking soda, and ½ teaspoon of salt. Set aside.

3. Cream Butter and Sugars: In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl with a hand mixer, beat the ¾ cup of room temperature butter, 1 cup of brown sugar, and ½ cup of granulated sugar on medium-high speed for 3-4 minutes. The mixture should be light, fluffy, and pale.

4. Add Eggs and Vanilla: Add the 3 room temperature eggs, one at a time, beating well for about 30 seconds after each one. Scrape down the sides of the bowl. Beat in the 2 teaspoons of vanilla extract.

5. Alternate Dry and Wet: With the mixer on low speed, add the dry flour mixture in three parts, alternating with the 1 cup of buttermilk in two parts. Begin and end with the dry ingredients (Flour -> Buttermilk -> Flour -> Buttermilk -> Flour). Mix only until just combined after each addition. Be careful not to over-mix.

6. Pour and Top: Pour the thick batter into the prepared 9×13 inch pan and spread it into an even layer. Sprinkle the prepared Pecan Crunch Topping evenly all over the surface of the batter.

Part 3: Bake the Cake and Prepare the Caramel Sauce

1. Bake: Place the cake in the preheated oven and bake for 35-45 minutes. The cake is done when the top is golden brown and a wooden skewer inserted into the center of the cake (avoiding the topping) comes out clean.

2. Make the Caramel Sauce: While the cake is baking, prepare the caramel sauce. In a medium saucepan over medium heat, melt the ½ cup of butter. Add the 1 cup of brown sugar and ½ cup of heavy cream. Whisk to combine.

3. Boil the Sauce: Bring the mixture to a boil, then reduce the heat to a simmer. Let it simmer gently, whisking occasionally, for 5-7 minutes until it has thickened slightly.

4. Finish the Sauce: Remove the saucepan from the heat and stir in the 1 teaspoon of vanilla and ½ teaspoon of salt. The sauce will be very hot. Set aside.

Part 4: Assemble the Final Cake

1. Poke and Pour: As soon as you take the cake out of the oven, while it is still hot, use the handle of a wooden spoon or a thick skewer to poke holes all over the surface of the cake, going about halfway down.

2. Drizzle with Caramel: Slowly and evenly pour about two-thirds of the warm caramel sauce over the hot cake. The sauce will seep into the holes and saturate the cake.

3. Cool and Serve: Let the cake cool in the pan for at least 30 minutes before slicing. Serve warm, with the remaining caramel sauce drizzled over each individual slice.