In a small bowl, combine the 1/4 cup milk and 3/4 tsp vinegar. Stir and set aside for 5 minutes to let the mixture thicken and curdle slightly (this creates a buttermilk substitute).

Preheat your oven to 350°. Line an 8x8-inch baking pan with parchment paper.

In a large mixing bowl, cream together the 1/2 cup butter and 3/4 cup sugar using an electric mixer until light and fluffy (about 2-3 minutes).

Add the 2 eggs one at a time, mixing well after each addition. Stir in the 2 tsp vanilla and 1 Tbsp pure maple syrup.

Add the 1/2 cup plain Greek yogurt and the milk-vinegar mixture, mixing until smooth.

In a separate bowl, whisk together the 2 cups flour, 1 Tbsp baking powder, and 1/2 tsp salt.

Gradually add the dry ingredients to the wet mixture, stir by hand just until combined. Do not overmix to avoid a dense cake.

In a small bowl, mix together the 1/3 cup brown sugar, 1 Tbsp cinnamon, and 1/2 cup flour. Add the 1/4 cup cold butter, cutting it in with a fork or pastry cutter until the mixture resembles coarse crumbs.

Pour half of the cake batter into the prepared pan and smooth the top.

Sprinkle half of the cinnamon/sugar mixture evenly over the batter.

Add the remaining cake batter on top, smoothing it out to cover the cinnamon layer. Use a knife to swirl the mixture into the batter slightly.

Sprinkle the remaining cinnamon/sugar mixture evenly over the top.

Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center of the cake comes out clean.

Allow the cake to cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.