Preheat oven to 325 degrees. Prepare a 12 cup muffin pan with nonstick spray. In large bowl, whisk together eggs, the 1/4 cup maple syrup and 3 tablespoons of the melted butter. In a medium bowl, stir together almond flour, baking soda and salt. Add dry ingredients to egg mixture; stir until just combined. In a small bowl, toss apples with sugar and 1 teaspoon of the cinnamon. Stir apple pieces into muffin batter. Using an ice cream scoop, fill muffin cups with batter. In a small bowl, stir remaining butter with the remaining maple syrup and remaining cinnamon. Pour a little over each muffin. Scatter almond slices over the muffins. Bake for 20-25 minutes, or until tops of muffins are firm. Cool in pan, on a rack for 10-15 minutes.