Preheat oven and liners:
Heat oven to 350°F (175°C) fan. Insert liners into a 6-cup jumbo muffin pan.Make flax egg:
In a small bowl, whisk 6 tsp ground flaxseed with 6 tsp water until a paste forms; let stand to thicken.Sift dry ingredients:
Sift 2 3/4 cups (370g) flour, 3 tsp baking powder, 1 tsp arrowroot, and 1 cup (180g) coconut sugar into a medium bowl. Stir to disperse any lumps.Mix wet and dry:
In another bowl, blend 1/4 cup (60g) yogurt, 1/2 cup (115g) softened butter, 1 1/4 cups (300ml) milk, and 1 tbsp caramel sauce. Add the wet mixture to the dry ingredients; fold until just combined. Do not overmix.Fill liners and top:
Fill each liner to the top with batter. Use a toothpick to swirl extra caramel on top, then generously cover with streusel.Bake muffins:
Bake on the center rack at 350°F (175°C) fan for 12 minutes. Lower the temperature to 320°F (160°C) fan and bake 16–18 minutes more, until a toothpick inserted in the center comes out clean.Cool and garnish:
Take the pan out of the oven and cool on a wire rack. Once cool, drizzle with additional caramel and, if desired, sprinkle with flaked sea salt. Remove from liners and serve.