Mexican Chocolate Streusel

In a food processor or a bowl, whirl or stir 1/2 cup all-purpose flour and 1/4 cup firmly packed brown sugar until well blended. Add 5 tablespoons butter and whirl or rub in with your fingers until mixture forms coarse crumbs. Pulse in 4 1/2 ounces (about 1 1/2 tablets) coarsely chopped Mexican chocolate (some larger chunks will remain), or chop finely with a knife and stir into flour mixture. Use immediately or chill airtight for up to 1 week.