In a medium bowl, combine 1 cup packed light brown sugar, 1 cup all-purpose flour, 2 teaspoons ground cinnamon, and 1 stick of softened unsalted butter. Use a fork or your fingers to mix until crumbly. Set aside.
In a large mixing bowl, beat together 2 sticks of softened unsalted butter and 1 1/4 cups granulated sugar until the mixture is light and fluffy. This should take about 3-5 minutes.
Add 5 large eggs, one at a time, mixing well after each addition. Then, stir in 1 tablespoon of vanilla extract.
In another bowl, whisk together 3 cups of all-purpose flour, 2 teaspoons of baking powder, and 1 teaspoon of salt.
Gradually add the dry mixture to the butter mixture, alternating with 1 cup of whole milk. Start and end with the dry ingredients. Mix until just combined. Be careful not to overmix.
Preheat your oven to 350°F (175°C). Grease your 9x13 inch baking pan. Pour half of the batter into the prepared pan, followed by half of the streusel mixture. Repeat with the remaining batter and streusel on top.
Bake in the preheated oven for 40-45 minutes or until a toothpick inserted in the center comes out clean.
Allow the cake to cool in the pan for about 15 minutes before transferring it to a cooling rack. Once cooled, slice and serve with your favorite coffee or tea.
