Preheat the oven to 325 F. Line a 9X13 baking pan with parchment paper that overhangs on at least 2 opposite sides. Set aside.

Whisk together the instant espresso and cream. Set aside.

Whisk together the flour and sea salt. Set aside.

Beat the eggs using a whip attachment (handheld mixer or stand mixer) until foamy and thickened. Add the sugar in a stream while running the mixer on medium low speed. Scrape the sides of the bowl down to completely incorporate the sugar. Then return to beating on medium high speed until the eggs are thick and pale yellow, about 3-5 minutes.

Beat in the vanilla and melted butter.

Add the flour mixture and mix in on low until mostly incorporated. Then finish folding by hand with a large spatula to completely incorporate the flour.

Remove 1 1/2 cups of this batter to a separate bowl.

Mix the espresso-cream mixture into the remaining batter in your original bowl. Spread this batter into the prepared pan.

Spoon the reserved 1 1/2 cups of "vanilla" batter over the coffee batter. Then use a butter knife or a skinny spatula to swirl the 2 together.

If desired, sprinkle the miniature chocolate chips over the top of the bars.

Bake for 20-25 minutes, until the center of the bars is set.

Cool completely then remove from the pan by the parchment paper overhang and slice.