Make the dough: Place the yeast and sugar in the bowl of a stand mixer fitted with a dough hook or paddle attachment. Or, if you do not own a stand mixer, a regular large mixing bowl. Heat the milk on the stove or in the microwave until warm to touch, about 110°F (43°C). Pour warm milk on top of yeast/sugar. Whisk gently to combine, then loosely cover with a clean kitchen towel and allow to sit for 5-10 minutes. The mixture will be frothy after 5-10 minutes.
If you do not have a mixer, you can mix by hand in this step. Add the butter, egg, flour, and salt. Beat on low speed for 4 minutes. Dough will be soft. Transfer it to a lightly floured work surface. Using lightly floured hands, knead it for 1 minute. If the dough is too sticky to handle, add 1-3 more Tablespoons of flour, but you want a very soft dough. Shape into a ball.
Place the dough in a greased bowl (I use nonstick spray to grease) and cover with plastic wrap or aluminum foil. Place in a slightly warm environment to rise until doubled in size, around 90 minutes.
Grease a 9-inch springform pan. Turn the risen dough out onto a lightly floured work surface, punch it down to release the air.
Shape the twist: Using a rolling pin, roll into a 12x16-inch rectangle. Spread jam evenly on top. If desired, dot randomly with raspberries. Tightly roll up the dough to form a 16-inch long log. Place the log on its seam. Using a sharp knife, cut the log in half lengthwise. I find a serrated knife works best. Criss-cross one half on top of the other half-- forming an X. Twist the two together then coil into a circle. Carefully transfer to prepared springform pan. (I carefully pick it up using two flat spatulas!) Cover the twist with plastic wrap or aluminum foil, then allow to rise once again in a slightly warm environment until puffy, about 45 minutes.
Adjust the oven rack to the lower third position then preheat oven to 350°F (177°C).
Bake until golden brown, about 40-45 minutes. If you find the top is browning too quickly, tent with aluminum foil. Remove from the oven and allow to cool for 5 minutes.
Make the vanilla icing: Whisk the icing ingredients together until smooth.
Carefully remove the rim of the springform pan. Drizzle with vanilla icing. Slice and serve. Cover and store leftovers in the refrigerator for up to 1 week.