In the bowl of a stand mixer, combine the warm water and cane sugar. Sprinkle the yeast over the top and let stand for 10 minutes to bloom.
Add the fine sea salt and 1 1/2 cups of flour. Beat on high for one minute with the paddle attachment. Switch to the hook attachment. Add the remainder of the flour, mixing on low to combine. Knead on low for 7 minutes. If the dough is very sticky to the touch, add an additional 1-2 tablespoons of flour.
Coat a clean bowl with a teaspoon of olive oil and place the focaccia dough in the bottom. It will be quite soft and a little bit sticky. Cover with a tea towel and let rise in a warm place for 1 hour or until doubled in bulk.
Preheat an oven to 425F. Generously grease a rimmed baking sheet with the remainder of the olive oil. Without punching down the dough, gently tip the risen focaccia dough onto the oiled pan. Use your fingers to press dimples in the flatbread, without deflating it too much. It should be about 1 1/2 inches thick all over. Let rest for 10 minutes.
Sprinkle flaky sea salt on top of the focaccia and the rosemary, if using. Bake on the top shelf of the oven, for about 22 minutes, or until light golden on top. Cool completely.
Slice focaccia into thick slabs. Generously spread with the radish ramp butter. Top with a few thinly sliced radishes, sprinkle with salt and serve at once.
Use any fun, coarse salt for the top. I use a red salt from Hawaii or a flaky Maldon salt.