Stir together the pecans, brown sugar, and cinnamon. Set aside.

Whisk together the flour, baking powder, and salt. Set aside.

Using an electric mixer on medium speed, mix the butter, sugar, eggs, and vanilla until thoroughly combined.

Reduce mixer speed to low. Add the flour mixture in three portions, alternating with two portions of buttermilk. Mix just until combined.

Scoop about a third of the batter to the prepared pan. Sprinkle the pecan mixture evenly over the batter. Scoop the remaining batter over the pecan mixture.

Bake 45 to 50 minutes, or until a pick inserted into the center comes out clean. Cool in the pan 15 minutes. Then transfer the cake to a wire rack to cool completely.

Stir together the confectioners' sugar, 1 tablespoon buttermilk, butter, and vanilla until smooth. If necessary, add the remaining buttermilk, about a teaspoon at a time, until the glaze is the desired consistency.

Pour or spoon the glaze over the cooled cake.