For the Grilled Steak:
Preheat grill to medium-high heat.
Brush steaks with olive oil and season generously with garlic, rosemary, salt, and black pepper.
Grill steaks for 4-6 minutes per side, depending on desired doneness.
Let the steaks rest for 5 minutes before serving.
For the Mocha Mousse:
Melt chocolate in a double boiler or microwave, stirring until smooth.
Dissolve coffee granules in 1 tbsp of hot water and stir into the melted chocolate.
In a separate bowl, whisk egg yolks and sugar until thick.
Gradually fold in the chocolate mixture to the egg mixture.
Whisk heavy cream in another bowl until stiff peaks form, then fold into the chocolate mixture.
Spoon mousse into serving dishes and chill for at least 2 hours before serving.