In a medium bowl, beat the first five ingredients together with an electric mixer until well combined. If the glaze starts to form peaks, add a little more milk. Set aside in a cool place.
In another bowl, mix together the flour, baking powder, and salt; set aside.
In a separate bowl, cream together the butter and sour cream until fluffy; add the sugar. Add eggs one at a time, then the vanilla. Gently fold in the flour mixture a little at a time until it is well combined.
Spread the batter into a 9x13-inch baking pan that has been lined with aluminum foil and liberally sprayed with cooking spray.
In a small bowl, mix the brown sugar, pecans, and cinnamon, cutting in the melted butter until the mixture is crumbly. Reserve.
Bake the cake in a preheated 350-degree oven for 20 minutes and apply half the glaze, followed by the pecan mixture. Bake for another 15-20 minutes or until a toothpick comes out clean.
Allow to cool in the pan for 10 minutes, then lift the foil out of the pan and place the cake on a wire rack. Remove the foil.
After 15 minutes, or just before serving, liberally drizzle with remaining glaze. Decorate with pecan halves if desired.