• Preheat oven to 375°F/190°C. Either spray the muffin tins with non-stick spray or use cupcake liners to line muffin cups in a muffin pan. Set aside.
• In a medium-sized mixing bowl, whisk the dry ingredients; all-purpose flour, baking powder, and salt together, set aside.
• In a large mixing bowl, using an electric hand mixer with beaters attached, cream butter and sugar until light and fluffy.
• To the mixing bowl, beat in the egg, vanilla extract, lemon zest, and juice of the lemon, then mix in the milk.
• Gently fold the dry ingredients into the wet ingredients until just combined.
• Fill the prepared muffin cups with the batter. Fill each cup about ⅔ the way full.

Make the topping

• In a small mixing bowl, combine brown sugar, flour, and lemon zest. Cut in butter with a fork or pastry cutter until crumbly
• Sprinkle the mixture over each muffin.
• Bake the muffins for 25 minutes, or until a toothpick inserted in the center of a muffin comes out clean.
• Cool slightly and serve!