Preheat oven to 350 degrees. Brush brioche triangles with melted butter and bake for 10 to 12 minutes, until golden.
Heat 2 tablespoons oil in a large skillet over medium-high heat. Saute onion in oil with salt, pepper, and bay leaves until onion becomes soft and golden, 8 to 10 minutes.
Add chicken livers to skillet and cook, stirring occasionally, until livers are browned and firm to the touch, 5 minutes.
Deglaze skillet with Marsala wine, scraping up brown bits, and reduce liquid by half.
Remove bay leaves and transfer liver-onion mixture to food processor; puree until smooth. With motor running, slowly add remaining olive oil. Adjust seasoning to taste, transfer to ramekins, cover, and chill at least four hours before serving. Serve with brioche crostini triangles.