1. In a large bowl set over a pan of barely simmering water, stir butter and unsweetened chocolate until melted and smooth. Remove from heat and whisk in brown sugar and granulated sugar. Add eggs one at a time, whisking well after each addition. Whisk in vanilla and almond extract. Stir in flour, cinnamon, baking powder, and salt until well blended.
2. Spread batter level in a buttered and floured 9- by 13-inch baking pan. Squeeze handfuls of Mexican chocolate streusel until it sticks together, then crumble into chunks evenly over surface of batter.
3. Bake brownies in a 325° oven until a wooden skewer inserted in the center comes out with moist crumbs attached, 30 to 35 minutes. Let cool in pan on a rack for at least 20 minutes, then cut into 24 or 32 squares (if making up to 1 day ahead, cool completely, then wrap uncut brownies airtight).
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.