Whisk together the flour, sugar, cardamom, and salt.
Add the butter and rub the ingredients between your hands until streusel crumbs form.
Place in the refrigerator.
Melt the honey and butter pieces in a saucepan over medium heat, stirring to combine.
Remove from heat. Let cool completely.
Grease and flour a 9” x 13” baking pan.
Trim and cut the rhubarb into 1 inch pieces
Preheat oven to 350 degrees F.
In a separate bowl, whisk together the all-purpose flour. baking powder, cinnamon, cardamom powder, and salt.
In a mixer, beat the eggs, sugar, and vanilla at high speed for 3 minutes.
Lower the speed to medium and slowly stream in the cooled, melted honey and butter combination.
Add the dry ingredients in thirds until combined completely.
Fold in the rhubarb pieces.
Spread the batter out onto the greased and floured pan. Smooth the top over with a spatula.
Evenly distribute the cardamom streusel over the top.
Bake the cake for 1 hour or until a knife inserted into the center of the cake comes out clean.
Remove the cake from the oven, and let cool completely before cutting into squares and serving.