To the pan, divide the batter to 9 muffins. I really like to use an ice cream scoop, as this creates well rounded muffins that have a fluffier dome and look to them. The batter will fill up high., this is correct and how you will get such such muffins.
Using your fingertips, crumble the streusel topping evenly over each muffin covering all of the batter well. Break the clumps into little pieces so all the batter is covered. Gently press any pieces that fall to the pan back into the streusel topping, gently, as we don't want to deflate the batter. This is important that the streusel sticks to the batter, otherwise if too much falls onto the pan, they can burn.
ALMOND FLOUR: It's crucial to use a superfine blanched almond flour because it is what makes the muffins light and fluffy. If it isn't fine, you will end up with more dense/wet muffins. I like the King Arthur flour, Wellbee's, Honeyville, Nature's Eats and Nut's.com brands best. They all work well.
GLUTEN-FREE VERSION: To make these gluten-free, you will still use the same amounts of almond flour given, but sub the 1 cup of spelt flour with 3/4 cup (96g) superfine oat flour and 1/4 cup (40g) brown rice flour. Instead of potato starch, you will sub with 4 1/2 tablespoons (36g) cornstarch). The reason you are using more cornstarch here is because oat flour can make things gummy, and the cornstarch helps to bind them (since no gluten) and make them fluffy. Everything else is the same. The batter is more runny than the spelt version, therefore divide the batter into 10 muffins instead of 9. The batter will fill up high in each muffin tin, this is normal and how you will end up with tall muffins. Check the muffins at 20 minutes for a clean toothpick, but they may take a couple of minutes longer, due to oven variance. Remember to use fine flours (I use Bob's Red Mill oat flour) for the best fluffy results.