Preheat oven to 350°F (175°C). Grease and flour a 9x13-inch baking dish.
Whisk flour, baking powder, and salt together in a medium bowl.
In a large bowl, cream butter and dark brown sugar with an electric mixer for 4 minutes until fluffy; beat in eggs one at a time, then add vanilla.
Add flour mixture alternately with whole milk in three batches, mixing on low speed until smooth.
Pour batter into prepared baking dish; bake for 30 to 35 minutes until a toothpick inserted into center comes out clean.
While warm, poke holes over the top of cake with the handle of a wooden spoon.
Warm butterscotch chips and heavy cream in microwave, stirring until smooth; pour warm butterscotch glaze over cake.
Sprinkle evenly with toasted chopped pecans and toffee bits; cool completely before slicing.