Preheat the oven to 180C/350F. Grease a 12-count muffin tin with muffin liners and set aside.
Start by preparing your streusel topping. Add all the ingredients into a mixing bowl, and mix together, until a crumble remains. Set aside.
In a small bowl, add your almond flour, baking powder, and salt and mix well. Set aside. In a separate bowl, add the sweetener, eggs, milk, and melted butter, and whisk together, until smooth and glossy.
Gently add the dry ingredients into the wet ingredients and fold them in, until smooth and combined. Distribute the muffin batter evenly amongst the muffin tin. Sprinkle with the crumble topping over each one.
Bake the muffins for 22-25 minutes, or until a skewer comes out clean. Remove from the oven and let cool in the muffin pan for 10 minutes, before transferring to a wire rack to cool completely.
Once cooled, make your icing by whisking together the powdered sugar with water. Drizzle over the top of each muffin.