If you want to bake your muffins right away, preheat the oven to 350F. Otherwise, wait until you’re ready to bake.
In a large bowl, whisk together the melted butter, brown sugar, eggs and vanilla. Whisk in the sour cream or yogurt. In another bowl, whisk together the flour, baking powder and salt. Add to the wet ingredients and stir (using a spatula this time) until almost combined; hold back a couple tablespoons of the pecans and Skor bits to add to the streusel topping and add the rest to the batter, stirring just until blended.
In a small bowl, blend the brown sugar, flour and butter for the streusel with a fork or your fingers. Blend in the reserved pecans and Skor bits.
If you like, cover and let your batter sit on the countertop for about an hour, or refrigerate for longer—even overnight. This will give them a finer texture and a little added lift—you may or may not notice much difference. Otherwise you can go ahead and bake them right away. Fill paper lined muffin cups so that they’re heaping (above the top) and sprinkle with the streusel mixture.
Bake for 25-35 minutes (how long they take will depend on their size), until golden, domed and springy to the touch.
Makes about 8-10 mmmuffins.