In a medium bowl, whisk together brown sugar, flour, cinnamon, nutmeg, and ⅛ teaspoon salt. Cut butter into ½-inch pieces and cut into the brown sugar mixture with a pastry cutter or your fingers. When the mixture resembles coarse meal, stir in Heath or Skor bar bits.
Preheat the oven to 350 degrees F.
In a large bowl, combine flour, baking powder, baking soda, salt and nutmeg. Combine butter and sugar in a large bowl, and blend with a mixer until fluffy. Add eggs, one at a time, beating well after each addition. Beat in vanilla extract.
Fold in the flour mixture alternately with yogurt, beginning and ending with the flour mixture. Do not overmix.
Coat 1½ muffin tins (18 cups) with cooking spray. Fill each cup one-third full with batter. Using half of the Heath bar mixture, generously top the batter. Fill with the rest of the batter and top with the remaining streusel.
Bake until the muffins are golden brown and a toothpick inserted into the center comes out clean, 15 to 18 minutes, rotating the pans from top to bottom and back to front halfway through baking.
Cool in the pan for 15 minutes. Remove the muffins from the pans and cool completely on a wire rack. Serve.