How to Make It
Preheat oven to 400°F. Lightly grease a 12-cup muffin pan with cooking spray or line with baking liners.
Melt 1 Tbsp. salted butter in a medium skillet over medium-high. Add diced apple and 2 Tbsp. granulated sugar to skillet; cook, stirring occasionally, until golden brown, 8 to 9 minutes. Scrape apple mixture into a medium bowl; cool 5 minutes.
Meanwhile, whisk together flour, brown sugar, baking powder, salt, cinnamon, and 3/4 cup toffee bits (such as Bits of Brickle) in a large bowl
Stir milk, melted butter, vanilla, and eggs into apple mixture; add to flour mixture, stirring until just combined. Divide batter evenly among prepared muffin cups. Sprinkle 1/2 cup toffee bits evenly over batter in muffin cups, pressing to adhere
Bake in preheated oven until a wooden pick inserted in center comes out clean, 18 to 19 minutes. Cool in pan 5 minutes; remove muffins to a wire rack, and let cool 15 minutes to serve warm. Or let cool to room temperature.