Add butter, allulose, and molasses to a bowl and beat on high speed with a hand mixer or stand mixer until smooth and creamy. Make sure to scrape down the sides as necessary to get it all incorporated.
Add remaining wet ingredients and beat on high until fully combined.
In a separate bowl, sift together all dry ingredients then mix with a whisk. Add dry ingredients to wet ingredients and mix on low speed or by hand until just combined. Cover and place in fridge for ~30 minutes.
Preheat oven to 425F / 220C, making sure oven rack is in the center position or one position lower than center (not too close to the top).
Add all crumble topping ingredients to a small mixing bowl and work them together with a pastry cutter or fork. (You can also use your fingers to help break up any clumps)
Add all frosting ingredients to a small bowl and mix with a hand mixer until very smooth. Cover and set aside.
Chop apples into small pieces (final amount should match what is in recipe), toss them in a small amount of flour, then fold them into the batter.
Fill muffin pan with liners then scoop ~90g of batter into each. (If you followed all measurements correctly, this will make 12 muffins, allowing for a little leftover on spoons and bowl).
Place into oven on center rack and bake for 5 minutes. Then reduce the temperature to 325F / 160C and bake for an additional 20-23 minutes or until a toothpick comes out clean. Remove from oven and let cool for 5 minutes before removing from pan and placing on a wire rack.
Drizzle with frosting (add a little more water if necessary) and and let the muffins continue to cool while the frosting firms up a bit.