Preheat oven to 350°F and grease a bundt pan well. Arrange pecans evenly in the bottom of the pan.
In a large bowl, cream butter, granulated sugar, and brown sugar until light and fluffy.
Add eggs one at a time, mixing well after each addition and scraping the bowl often.
In a separate bowl, whisk flour, baking powder, baking soda, and salt.
Add dry ingredients and buttermilk to the batter in thirds, starting and ending with flour. Mix on low speed.
Gently fold batter to ensure everything is evenly mixed.
Spoon a small layer of batter over pecans, then add remaining batter and smooth the top.
Bake for 60–70 minutes, until golden and a tester comes out with moist crumbs.
Cool cake completely in the pan, then turn out onto a plate.
For the glaze, cook butter, brown sugar, heavy cream, salt, and vanilla over medium heat until smooth.
Pour glaze slowly over the cooled cake, letting it set between pours.