Place a rack in the center of the oven and heat to 375°F. Line a 12-cup standard muffin tin with paper liners.

Melt the butter in a small skillet or saucepan over medium heat. Cook for about 5 minutes, stirring occasionally, scraping milk solids off the bottom and sides of the pan, until the milk solids turn golden brown and smell nutty. Pour the butter and brown bits into a large bowl and cool for 10 minutes. Remove 3 tablespoons of the butter and reserve in a small bowl.

Add the sour cream, applesauce and brown sugar and whisk until smooth. Whisk in the eggs, vanilla, cinnamon and salt and whisk until smooth. Whisk in the baking powder and soda until combined. Then fold in the flour until combined. Fold in diced apples and toffee until combined.

Make topping: Add the sugar, flour and cinnamon to the small bowl with the reserved browned butter and mix with fingers until crumbs form.

Spoon batter into muffin liners; they will be nearly full. Top evenly with topping mixture. Sprinkle tops with sea salt.

Bake until the muffins until puffed and golden, and a toothpick inserted into the center comes out clean, about 15 to 20 minutes. Let the muffins cool in the pan for 10 minutes, then remove to a rack to cool completely.

Store leftover muffins at room temperature in an airtight container for up to 4 days. Or freeze for up to 4 months.