Preheat oven to 350°F. Grease and flour a Bundt pan; set aside.
PREPARE THE STREUSEL:
In a medium bowl, combine the chopped pecans, flour, brown sugar, cinnamon, nutmeg and salt. Add chilled butter and mix together with your fingers until it forms a crumble. It should have the texture of wet sand, and you shouldn’t see any dry pockets of flour. Set aside while you prepare the cake batter.
PREPARE THE CAKE:
Combine cake mix, dry pudding mix, oil, water and eggs in a large bowl. Use an electric mixer to beat on low speed for 30 seconds. Increase the speed to medium-high and beat for 2 more minutes, scraping the sides of the bowl if necessary.
Sprinkle half of the streusel mixture in the prepared pan (this will be the top of your cake when it’s later inverted after baking). Top with half of the batter. Add remaining streusel, and then remaining batter.
BAKE THE CAKE:
Bake for 50 minutes, or until a toothpick inserted in the center comes out clean. Cool cake in pan on a wire rack for 15 minutes; invert cake onto wire rack, and let cool completely before glazing (about 2 hours).
GLAZE THE CAKE:
Once the cake is cool, prepare the glaze. In a small bowl, whisk together the confectioners’ sugar, milk and vanilla extract until smooth. Drizzle glaze over top of the cake.
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Notes
Chop the nuts very fine so that they blend into the streusel easily. You don't want them ground into a powder, but small bits are ideal. I like to buy chopped pecans and then pulse them a couple of times in a mini food processor to create slightly smaller pieces, or purchase a package of "chipped pecans."
A light-colored or gold Bundt pan is always best to ensure even baking and color. Dark metal pans tend to cause the exterior of the cake to burn before the center is cooked through.
Grease the Bundt pan really well by coating with butter and then dusting with flour. Alternatively, you can use a cooking spray that's specifically made for baking (like this one). These options will ensure that your cake pops right out of the pan when it's done!
Don't leave the cake in the pan for too long after baking -- about 10-15 minutes is plenty. After that, condensation will start to form and the moisture will negatively impact the crust.
The cake gets better as it sits, so this is a nice option to prepare the day before you plan to serve it.