Heat oven to 350°F. Combine sugar and 3/4 cup butter in large bowl. Beat at medium speed, scraping bowl often, until well mixed. Add eggs and vanilla; continue beating until creamy.
Combine 2-3/4 cups, baking powder, baking soda and salt in another medium bowl. Add to butter mixture alternately with sour cream, mixing well after each addition. Toss together rhubarb, strawberries and remaining 1/4 cup flour in same medium bowl; gently stir into batter. Spread evenly into greased 13x9-inch baking pan.
Combine brown sugar, 1/4 cup flour and cinnamon in small bowl, cut in 1/4 cup butter until mixture resembles coarse crumbs. Sprinkle evenly over batter. Bake for 45 - 55 minutes.
In a medium bowl, combine all purpose flour, baking powder, salt, baking soda and ground nutmeg. Set aside.
In a large pouring vessel combine whole milk, buttermilk, sweet potato puree and vanilla extract. Set aside.
In a large bowl, cream unsalted butter and brown sugar with an electric mixer, until fluffy. Add eggs, one at a time, beating well after each addition. Alternately mix in dry and wet ingredients into the butter, starting and ending with the dry; mix only until each addition is incorporated, DO NOT OVERMIX.
Coat a muffin pan with nonstick spray and fill muffin cups 1/2 full. Bake in a 350 degree oven for 20-25 minutes, or until a toothpick inserted into the center of the muffin comes out clean.
While muffins are baking, in a shallow bowl, combine granulated sugar, ground cinnamon and ground nutmeg. In a separate bowl, melt 1 stick of unsalted butter.
When the muffins are cool enough to handle, yet still warm, remove muffins from the pan, brush them completely with the melted butter, and roll in the sugar mixture to coat.