Would you like a sandwich with your coffee?

Starbucks (NASDAQ:SBUX) just booked strong revenue growth for its fiscal second quarter. And the rise was fueled in part by one unlikely star product: panini sandwiches.

The coffee king's sales jumped higher by 7% in the U.S. Of course, popular espresso beverages -- like the vanilla spice latte and hazelnut macchiato -- did their part by selling well. But food also played a big role in delivering Starbucks' outsized sales gains.

Food and prepaid cards pair well
That's partly thanks to the company's prior success in selling massive amounts of prepaid cards. Customers loaded more than $1 billion onto their loyalty cards in the final three months of 2012, and gift cards were one of the most popular presents over the holidays. Those trends added up to billions of dollars of Starbucks money just sitting in people's pockets, ready to be spent.

And customers chose to spend more of that cash on food last quarter. The company's expanded availability of paninis was a hit, boosting food sales and drawing more traffic into the stores during those typically slower afternoon hours.

Satisfied customers
Thankfully for Starbucks, the uptick in food sales didn't seem to hurt customer satisfaction, either. Speed and friendliness metrics have fallen at McDonald's (NYSE:MCD) lately, and customer complaints are on the rise. It's probably no coincidence that Mickey D's has seen its sales growth tick down over the past few quarters.

But Starbucks has managed the opposite result. The company said that, despite the busier stores and expanded food options, order accuracy and staff friendliness metrics actually rose. The overall satisfaction scores at Starbucks locations improved last quarter by the highest amount in two years.

Another helping
All of that bodes well for the company's ambitious plans to broaden its food menu. Starbucks spent $100 million to buy the La Boulange bakery brand last year, and it has been testing new bakery food options in its Northern California stores.

The company is happy enough with the results that it expects to introduce the new treats to all Starbucks locations over the next 18 months. Given that customers added 32% more dollars onto their loyalty cards last quarter, they'll have plenty of money to spend on trying out the new food options when they arrive. 

Fool contributor Demitrios Kalogeropoulos owns shares of McDonald's. The Motley Fool recommends and owns shares of McDonald's and Starbucks. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.