In a large bowl, whisk together the flour, salt, sugar, and instant yeast.
Add the lukewarm water and mix with a rubber spatula until a sticky dough forms.
Lightly coat the surface of the dough with 2 tablespoons of vegetable oil.
Cover the bowl with a cloth or plastic wrap and let the dough rise at room temperature for about 2 hours, or until doubled in size.
Deflate the dough by gently pulling it toward the center using damp hands or two forks, rotating the bowl as you go to form a rough ball.
Let the dough rest for an additional 3 to 4 hours.
Transfer the dough into a greased pan, fold, and gently dimple the surface with your fingers.
Bake at 475°F until the focaccia is golden brown, about 20-25 minutes. Remove from the oven and let it cool.
Place the jelly into a plastic bag, snip off the tip, and pipe it into the focaccia by poking holes in the top.
For the icing, mix 1 cup powdered sugar, 2 tablespoons milk, and 1/2 teaspoon vanilla extract until smooth. Drizzle over the focaccia.
